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SubjectRe: [PATCH v5 2/3] seccomp_filters: system call filtering using BPF
From
Hello,

Feedback below. I know you send out v6, but I'm not going to play
eternal catch-up, so here it is.

On Sat, January 28, 2012 00:24, Will Drewry wrote:
> [This patch depends on luto@mit.edu's no_new_privs patch:
> https://lkml.org/lkml/2012/1/12/446
> ]
>
> This patch adds support for seccomp mode 2. This mode enables dynamic
> enforcement of system call filtering policy in the kernel as specified
> by a userland task. The policy is expressed in terms of a Berkeley
> Packet Filter program, as is used for userland-exposed socket filtering.
> Instead of network data, the BPF program is evaluated over struct
> seccomp_filter_data at the time of the system call.
>
> A filter program may be installed by a userland task by calling
> prctl(PR_ATTACH_SECCOMP_FILTER, &fprog);
> where fprog is of type struct sock_fprog.
>
> If the first filter program allows subsequent prctl(2) calls, then
> additional filter programs may be attached. All attached programs
> must be evaluated before a system call will be allowed to proceed.
>
> To avoid CONFIG_COMPAT related landmines, once a filter program is
> installed using specific is_compat_task() value, it is not allowed to
> make system calls using the alternate entry point.
>
> Filter programs will be inherited across fork/clone and execve.
> However, if the task attaching the filter is unprivileged
> (!CAP_SYS_ADMIN) the no_new_privs bit will be set on the task. This
> ensures that unprivileged tasks cannot attach filters that affect
> privileged tasks (e.g., setuid binary).
>
> There are a number of benefits to this approach. A few of which are
> as follows:
> - BPF has been exposed to userland for a long time.
> - Userland already knows its ABI: system call numbers and desired
> arguments
> - No time-of-check-time-of-use vulnerable data accesses are possible.
> - system call arguments are loaded on demand only to minimize copying
> required for system call number-only policy decisions.
>
> This patch includes its own BPF evaluator, but relies on the
> net/core/filter.c BPF checking code. It is possible to share
> evaluators, but the performance sensitive nature of the network
> filtering path makes it an iterative optimization which (I think :) can
> be tackled separately via separate patchsets. (And at some point sharing
> BPF JIT code!)
>
> v5: - uses syscall_get_arguments
> (indan@nul.nu,oleg@redhat.com, mcgrathr@chromium.org)
> - uses union-based arg storage with hi/lo struct to
> handle endianness. Compromises between the two alternate
> proposals to minimize extra arg shuffling and account for
> endianness assuming userspace uses offsetof().

Why would user space use offsetof()? Do you mean when assembling the BPF
code? So this basically means 32-bit and 64-bit filters are different,
even when they could be the same. Hm.

> (mcgrathr@chromium.org, indan@nul.nu)
> - update Kconfig description
> - add include/seccomp_filter.h and add its installation
> - (naive) on-demand syscall argument loading
> - drop seccomp_t (eparis@redhat.com)
> - adds proper compat prctl call copying
> v4: - adjusted prctl to make room for PR_[SG]ET_NO_NEW_PRIVS
> - now uses current->no_new_privs
> (luto@mit.edu,torvalds@linux-foundation.com)
> - assign names to seccomp modes (rdunlap@xenotime.net)
> - fix style issues (rdunlap@xenotime.net)
> - reworded Kconfig entry (rdunlap@xenotime.net)
> v3: - macros to inline (oleg@redhat.com)
> - init_task behavior fixed (oleg@redhat.com)
> - drop creator entry and extra NULL check (oleg@redhat.com)
> - alloc returns -EINVAL on bad sizing (serge.hallyn@canonical.com)
> - adds tentative use of "always_unprivileged" as per
> torvalds@linux-foundation.org and luto@mit.edu
> v2: - (patch 2 only)
>
> Signed-off-by: Will Drewry <wad@chromium.org>
> ---
> include/linux/Kbuild | 1 +
> include/linux/prctl.h | 3 +
> include/linux/seccomp.h | 63 ++++
> include/linux/seccomp_filter.h | 79 +++++

Just stuff it into seccomp.h? If this is added, then this will be seccomp,
and the old mode can be seen as a standard, very restrictive filter.

> kernel/Makefile | 1 +
> kernel/fork.c | 4 +
> kernel/seccomp.c | 10 +-
> kernel/seccomp_filter.c | 620 ++++++++++++++++++++++++++++++++++++++++

Same here. There isn't much to seccomp.c, why not add the filter code there?

If you insist on keeping it separate, then why call it seccomp at all?
Just call it syscall_filter or something.

> kernel/sys.c | 4 +
> security/Kconfig | 20 ++
> 10 files changed, 804 insertions(+), 1 deletions(-)

What is the difference in kernel binary size with and without this?
If it's too big I think the BPF filtering code should be shared with
the networking version from the start.

Actually, that is something that is wanted anyway, so why not use the
networking code from the start? That should reduce the code amount a
lot. So I think if this gets merged the consolidated version should be
merged. Only downside would be that networking support is needed for
this feature, but without networking there shouldn't be much need for
this feature anyway.

> create mode 100644 include/linux/seccomp_filter.h
> create mode 100644 kernel/seccomp_filter.c
>
> diff --git a/include/linux/Kbuild b/include/linux/Kbuild
> index c94e717..5659454 100644
> --- a/include/linux/Kbuild
> +++ b/include/linux/Kbuild
> @@ -330,6 +330,7 @@ header-y += scc.h
> header-y += sched.h
> header-y += screen_info.h
> header-y += sdla.h
> +header-y += seccomp_filter.h
> header-y += securebits.h
> header-y += selinux_netlink.h
> header-y += sem.h
> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
> index 7ddc7f1..b8c4beb 100644
> --- a/include/linux/prctl.h
> +++ b/include/linux/prctl.h
> @@ -114,4 +114,7 @@
> # define PR_SET_MM_START_BRK 6
> # define PR_SET_MM_BRK 7
>
> +/* Set process seccomp filters */
> +#define PR_ATTACH_SECCOMP_FILTER 37
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index 171ab66..3992bb6 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -5,10 +5,29 @@
> #ifdef CONFIG_SECCOMP
>
> #include <linux/thread_info.h>
> +#include <linux/types.h>
> #include <asm/seccomp.h>
>
> +/* Valid values of seccomp_struct.mode */
> +#define SECCOMP_MODE_DISABLED 0 /* seccomp is not in use. */
> +#define SECCOMP_MODE_STRICT 1 /* uses hard-coded seccomp.c rules. */
> +#define SECCOMP_MODE_FILTER 2 /* system call access determined by filter. */
> +
> +struct seccomp_filter;
> +/**
> + * struct seccomp_struct - the state of a seccomp'ed process
> + *
> + * @mode: indicates one of the valid values above for controlled
> + * system calls available to a process.
> + * @filter: Metadata for filter if using CONFIG_SECCOMP_FILTER.
> + * @filter must only be accessed from the context of current as there
> + * is no guard.
> + */
> struct seccomp_struct {
> int mode;
> +#ifdef CONFIG_SECCOMP_FILTER
> + struct seccomp_filter *filter;
> +#endif

Why a separate config option for this? Just depend on SECCOMP and networking?

> };
>
> extern void __secure_computing(int);
> @@ -51,4 +70,48 @@ static inline int seccomp_mode(struct seccomp_struct *s)
>
> #endif /* CONFIG_SECCOMP */
>
> +#ifdef CONFIG_SECCOMP_FILTER
> +
> +
> +extern long prctl_attach_seccomp_filter(char __user *);
> +
> +extern struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *);
> +extern void put_seccomp_filter(struct seccomp_filter *);
> +
> +extern int seccomp_test_filters(int);
> +extern void seccomp_filter_log_failure(int);
> +extern void seccomp_struct_fork(struct seccomp_struct *child,
> + const struct seccomp_struct *parent);

Lousy function name, what is it doing?

> +
> +static inline void seccomp_struct_init_task(struct seccomp_struct *seccomp)
> +{
> + seccomp->mode = SECCOMP_MODE_DISABLED;
> + seccomp->filter = NULL;
> +}

This doesn't actually do anything, just get rid of it and/or adapt seccomp_struct_fork()?

And it seems wrong to do this at fork time considering that the mode is supposed
to keep the mode across forks, so it seems safer to update it to the correct
values immediately.

> +
> +/* No locking is needed here because the task_struct will
> + * have no parallel consumers.
> + */
> +static inline void seccomp_struct_free_task(struct seccomp_struct *seccomp)
> +{
> + put_seccomp_filter(seccomp->filter);
> + seccomp->filter = NULL;
> +}

How many functions do you need to free one object?

I'm sure you can get rid of a couple, starting with this one. Just use
put_seccomp_filter() directly, the NULL is useless as task_struct is
going away.

> +
> +#else /* CONFIG_SECCOMP_FILTER */
> +
> +#include <linux/errno.h>
> +
> +struct seccomp_filter { };
> +/* Macros consume the unused dereference by the caller. */
> +#define seccomp_struct_init_task(_seccomp) do { } while (0);
> +#define seccomp_struct_fork(_tsk, _orig) do { } while (0);
> +#define seccomp_struct_free_task(_seccomp) do { } while (0);
> +
> +static inline long prctl_attach_seccomp_filter(char __user *a2)
> +{
> + return -ENOSYS;
> +}
> +
> +#endif /* CONFIG_SECCOMP_FILTER */
> #endif /* _LINUX_SECCOMP_H */
> diff --git a/include/linux/seccomp_filter.h b/include/linux/seccomp_filter.h
> new file mode 100644
> index 0000000..3ecd641
> --- /dev/null
> +++ b/include/linux/seccomp_filter.h
> @@ -0,0 +1,79 @@
> +/*
> + * Secomp-based system call filtering data structures and definitions.
> + *
> + * Copyright (C) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>

(Not my expertise, but I'm not sure if that makes legal sense.)

> + *
> + * This copyrighted material is made available to anyone wishing to use,
> + * modify, copy, or redistribute it subject to the terms and conditions
> + * of the GNU General Public License v.2.
> + *
> + */
> +
> +#ifndef __LINUX_SECCOMP_FILTER_H__
> +#define __LINUX_SECCOMP_FILTER_H__
> +
> +#include <asm/byteorder.h>
> +#include <linux/compiler.h>
> +#include <linux/types.h>
> +
> +/*
> + * Keep the contents of this file similar to linux/filter.h:
> + * struct sock_filter and sock_fprog and versions.
> + * Custom naming exists solely if divergence is ever needed.
> + */
> +
> +/*
> + * Current version of the filter code architecture.
> + */
> +#define SECCOMP_BPF_MAJOR_VERSION 1
> +#define SECCOMP_BPF_MINOR_VERSION 1

If it's a fork of the current networking code it should follow their version
numbering too. But these defines aren't used anywhere..?

> +
> +struct seccomp_filter_block { /* Filter block */
> + __u16 code; /* Actual filter code */
> + __u8 jt; /* Jump true */
> + __u8 jf; /* Jump false */
> + __u32 k; /* Generic multiuse field */
> +};
> +
> +struct seccomp_fprog { /* Required for SO_ATTACH_FILTER. */
> + unsigned short len; /* Number of filter blocks */
> + struct seccomp_filter_block __user *filter;
> +};
> +
> +/* Ensure the u32 ordering is consistent with platform byte order. */
> +#if defined(__LITTLE_ENDIAN)
> +#define SECCOMP_ENDIAN_SWAP(x, y) x, y
> +#elif defined(__BIG_ENDIAN)
> +#define SECCOMP_ENDIAN_SWAP(x, y) y, x
> +#else
> +#error edit for your odd arch byteorder.
> +#endif
> +
> +/* System call argument layout for the filter data. */
> +union seccomp_filter_arg {
> + struct {
> + __u32 SECCOMP_ENDIAN_SWAP(lo32, hi32);
> + };
> + __u64 u64;
> +};

Ugh. What was the advantage of doing this again? This looks like optimising
at the wrong level to me.

It doesn't make the ABI simpler, it doesn't make writing filters easier,
it doesn't make your code nicer, and I really doubt it makes it run
anything faster either. It's just obfuscating the code.

If you don't want to hide endianness then don't provide those lo32 and hi32
structure members and just make it a u64. That's at least very clear. But
swapping them around depending on endianness is not good, because it's only
confusing things.

But if you have to do it, then at least do:

#if defined(__LITTLE_ENDIAN)
union seccomp_filter_arg {
struct {
__u32 lo32;
__u32 hi32;
};
__u64 u64;
};
#else
union seccomp_filter_arg {
struct {
__u32 hi32;
__u32 lo32;
};
__u64 u64;
};
#endif

Instead of hiding what you're doing.

> +
> +/*
> + * Expected data the BPF program will execute over.
> + * Endianness will be arch specific, but the values will be
> + * swapped, as above, to allow for consistent BPF programs.

So now what? You need platform dependent BPF programs depending on
the arch's endianness? Just because you want to stuff one 64-bit
longs directly into two 32-bit ints? That saves how much, two
instructions?

> + */
> +struct seccomp_filter_data {
> + int syscall_nr;
> + __u32 __reserved;
> + union seccomp_filter_arg args[6];
> +};
> +
> +#undef SECCOMP_ENDIAN_SWAP
> +
> +/*
> + * Defined valid return values for the BPF program.
> + */
> +#define SECCOMP_BPF_ALLOW 0xFFFFFFFF
> +#define SECCOMP_BPF_DENY 0

This doesn't make sense. Please be consistent with C and swap it around.

> +
> +#endif /* __LINUX_SECCOMP_FILTER_H__ */
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 2d9de86..fd81bac 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -78,6 +78,7 @@ obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
> obj-$(CONFIG_LOCKUP_DETECTOR) += watchdog.o
> obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
> obj-$(CONFIG_SECCOMP) += seccomp.o
> +obj-$(CONFIG_SECCOMP_FILTER) += seccomp_filter.o
> obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
> obj-$(CONFIG_TREE_RCU) += rcutree.o
> obj-$(CONFIG_TREE_PREEMPT_RCU) += rcutree.o
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 051f090..f312edb 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -34,6 +34,7 @@
> #include <linux/cgroup.h>
> #include <linux/security.h>
> #include <linux/hugetlb.h>
> +#include <linux/seccomp.h>
> #include <linux/swap.h>
> #include <linux/syscalls.h>
> #include <linux/jiffies.h>
> @@ -169,6 +170,7 @@ void free_task(struct task_struct *tsk)
> free_thread_info(tsk->stack);
> rt_mutex_debug_task_free(tsk);
> ftrace_graph_exit_task(tsk);
> + seccomp_struct_free_task(&tsk->seccomp);
> free_task_struct(tsk);
> }
> EXPORT_SYMBOL(free_task);
> @@ -1093,6 +1095,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> goto fork_out;
>
> ftrace_graph_init_task(p);
> + seccomp_struct_init_task(&p->seccomp);

If you just do a get on the filter's krefs here then cleanup is done correctly
by free_task(). So just call seccomp_struct_fork() or get_seccomp_filter() here.

>
> rt_mutex_init_task(p);
>
> @@ -1376,6 +1379,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> if (clone_flags & CLONE_THREAD)
> threadgroup_change_end(current);
> perf_event_fork(p);
> + seccomp_struct_fork(&p->seccomp, &current->seccomp);

This can be moved above, before the first error goto.

>
> trace_task_newtask(p, clone_flags);
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index e8d76c5..a045dd4 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -37,7 +37,7 @@ void __secure_computing(int this_syscall)
> int * syscall;
>
> switch (mode) {
> - case 1:
> + case SECCOMP_MODE_STRICT:
> syscall = mode1_syscalls;
> #ifdef CONFIG_COMPAT
> if (is_compat_task())
> @@ -48,6 +48,14 @@ void __secure_computing(int this_syscall)
> return;
> } while (*++syscall);
> break;
> +#ifdef CONFIG_SECCOMP_FILTER
> + case SECCOMP_MODE_FILTER:
> + if (seccomp_test_filters(this_syscall) == 0)

Get rid of the '== 0' and have proper return values instead.

> + return;
> +

No need for an empty line here.

> + seccomp_filter_log_failure(this_syscall);
> + break;
> +#endif
> default:
> BUG();
> }
> diff --git a/kernel/seccomp_filter.c b/kernel/seccomp_filter.c
> new file mode 100644
> index 0000000..e57219e
> --- /dev/null
> +++ b/kernel/seccomp_filter.c
> @@ -0,0 +1,620 @@
> +/*
> + * linux/kernel/seccomp_filter.c
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + *
> + * Copyright (C) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
> + *
> + * Extends linux/kernel/seccomp.c to allow tasks to install system call
> + * filters using a Berkeley Packet Filter program which is executed over
> + * struct seccomp_filter_data.
> + */
> +
> +#include <asm/syscall.h>
> +
> +#include <linux/capability.h>
> +#include <linux/compat.h>
> +#include <linux/err.h>
> +#include <linux/errno.h>
> +#include <linux/rculist.h>
> +#include <linux/filter.h>
> +#include <linux/kallsyms.h>
> +#include <linux/kref.h>
> +#include <linux/module.h>
> +#include <linux/pid.h>
> +#include <linux/prctl.h>
> +#include <linux/ptrace.h>
> +#include <linux/ratelimit.h>
> +#include <linux/reciprocal_div.h>
> +#include <linux/regset.h>
> +#include <linux/seccomp.h>
> +#include <linux/seccomp_filter.h>
> +#include <linux/security.h>
> +#include <linux/seccomp.h>
> +#include <linux/sched.h>
> +#include <linux/slab.h>
> +#include <linux/uaccess.h>
> +#include <linux/user.h>
> +
> +
> +/**
> + * struct seccomp_filter - container for seccomp BPF programs
> + *
> + * @usage: reference count to manage the object lifetime.
> + * get/put helpers should be used when accessing an instance
> + * outside of a lifetime-guarded section. In general, this
> + * is only needed for handling filters shared across tasks.
> + * @parent: pointer to the ancestor which this filter will be composed with.
I guess the code will be clearer than the comment. Parent task or filter?
Is it a reverse linked list or something? I'm confused.

> + * @insns: the BPF program instructions to evaluate
> + * @count: the number of instructions in the program.
> + *
> + * seccomp_filter objects should never be modified after being attached
> + * to a task_struct (other than @usage).
> + */
> +struct seccomp_filter {
> + struct kref usage;
> + struct seccomp_filter *parent;
> + struct {
> + uint32_t compat:1;
> + } flags;

This flag isn't documented above.

> + unsigned short count; /* Instruction count */

Why a short?

> + struct sock_filter insns[0];

Newer gcc supports insns[], but I don't know what the kernel convention is.

> +};
> +
> +/*
> + * struct seccomp_filter_metadata - BPF data wrapper
> + * @data: data accessible to the BPF program.
> + * @has_args: indicates that the args have been lazily populated.
> + *
> + * Used by seccomp_load_pointer.
> + */
> +struct seccomp_filter_metadata {
> + struct seccomp_filter_data data;
> + bool has_args;
> +};

Why use 'bool' instead of a nifty struct { uint32_t : 1 }?
It seems that has_args is something that should be stored elsewhere,
it doesn't seem to warrant its own structure.

> +
> +static unsigned int seccomp_run_filter(void *, uint32_t,
> + const struct sock_filter *);

It almosts fits, just put it on one line.

> +
> +/**
> + * seccomp_filter_alloc - allocates a new filter object
> + * @padding: size of the insns[0] array in bytes
> + *
> + * The @padding should be a multiple of
> + * sizeof(struct sock_filter).

It's not padding, why call it such? Why not 'count' or 'bpf_blocks'?
That would reduce the sizeof(struct sock_filter)s to one.

> + *
> + * Returns ERR_PTR on error or an allocated object.
> + */
> +static struct seccomp_filter *seccomp_filter_alloc(unsigned long padding)
> +{
> + struct seccomp_filter *f;
> + unsigned long bpf_blocks = padding / sizeof(struct sock_filter);
> +
> + /* Drop oversized requests. */
> + if (bpf_blocks == 0 || bpf_blocks > BPF_MAXINSNS)
> + return ERR_PTR(-EINVAL);
> +
> + /* Padding should always be in sock_filter increments. */
> + if (padding % sizeof(struct sock_filter))
> + return ERR_PTR(-EINVAL);

...then this isn't needed.

> +
> + f = kzalloc(sizeof(struct seccomp_filter) + padding, GFP_KERNEL);
> + if (!f)
> + return ERR_PTR(-ENOMEM);
> + kref_init(&f->usage);
> + f->count = bpf_blocks;
> + return f;
> +}
> +
> +/**
> + * seccomp_filter_free - frees the allocated filter.
> + * @filter: NULL or live object to be completely destructed.
> + */
> +static void seccomp_filter_free(struct seccomp_filter *filter)
> +{
> + if (!filter)
> + return;
> + put_seccomp_filter(filter->parent);

Why do put on the parent here?

This function is only called once below by __put_seccomp_filter, perhaps merge them?

> + kfree(filter);
> +}
> +
> +static void __put_seccomp_filter(struct kref *kref)
> +{
> + struct seccomp_filter *orig =
> + container_of(kref, struct seccomp_filter, usage);
> + seccomp_filter_free(orig);

I find it unexpected that __put_seccomp_filter() calls put_seccomp_filter(),
why have two versions then? I have the feeling all this can be streamlined
more.

> +}
> +
> +void seccomp_filter_log_failure(int syscall)
> +{
> + pr_info("%s[%d]: system call %d blocked at 0x%lx\n",
> + current->comm, task_pid_nr(current), syscall,
> + KSTK_EIP(current));
> +}
> +
> +/* put_seccomp_filter - decrements the ref count of @orig and may free. */
> +void put_seccomp_filter(struct seccomp_filter *orig)
> +{
> + if (!orig)
> + return;
> + kref_put(&orig->usage, __put_seccomp_filter);
> +}
> +
> +/* get_seccomp_filter - increments the reference count of @orig. */
> +struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig)
> +{
> + if (!orig)
> + return NULL;
> + kref_get(&orig->usage);
> + return orig;
> +}

This function should be merged with seccomp_struct_fork(), it's pretty much only
called there.

> +
> +#if BITS_PER_LONG == 32
> +static inline unsigned long *seccomp_filter_data_arg(
> + struct seccomp_filter_data *data, int index)
> +{
> + /* Avoid inconsistent hi contents. */
> + data->args[index].hi32 = 0;
> + return (unsigned long *) &(data->args[index].lo32);
> +}
> +#elif BITS_PER_LONG == 64
> +static inline unsigned long *seccomp_filter_data_arg(
> + struct seccomp_filter_data *data, int index)
> +{
> + return (unsigned long *) &(data->args[index].u64);

No need for any casts, is there? And this looks a bit untidy.


> +}
> +#else
> +#error Unknown BITS_PER_LONG.
> +#endif
> +
> +/**
> + * seccomp_load_pointer: checks and returns a pointer to the requested offset
> + * @buf: u8 array to index into
> + * @buflen: length of the @buf array
> + * @offset: offset to return data from
> + * @size: size of the data to retrieve at offset
> + * @unused: placeholder which net/core/filter.c uses for for temporary
> + * storage. Ideally, the two code paths can be merged.

The documentation doesn't matches the actual code?

> + *
> + * Returns a pointer to the BPF evaluator after checking the offset and size
> + * boundaries.
> + */
> +static inline void *seccomp_load_pointer(void *data, int offset, size_t size,
> + void *buffer)
> +{
> + struct seccomp_filter_metadata *metadata = data;
> + int arg;
> + if (offset >= sizeof(metadata->data))
> + goto fail;
> + if (offset < 0)
> + goto fail;

Perhaps use unsigned offset?

> + if (size > sizeof(metadata->data) - offset)
> + goto fail;
> + if (metadata->has_args)
> + goto pass;
> + /* No argument data touched. */
> + if (offset + size - 1 < offsetof(struct seccomp_filter_data, args))
> + goto pass;
> + for (arg = 0; arg < ARRAY_SIZE(metadata->data.args); ++arg)
> + syscall_get_arguments(current, task_pt_regs(current), arg, 1,
> + seccomp_filter_data_arg(&metadata->data, arg));
> + metadata->has_args = true;

Wasn't the idea that arguments would be loaded on demand?

And as arguments are usually only checked once and not twice, caching the
values isn't really needed. So perhaps get rid of the (meta)data structure
and just have a switch that returns the right data directly depending on
the offset? It seems the over all code would be simpler that way.

> +pass:
> + return ((__u8 *)(&metadata->data)) + offset;
> +fail:
> + return NULL;
> +}
> +
> +/**
> + * seccomp_test_filters - tests 'current' against the given syscall
> + * @syscall: number of the system call to test
> + *
> + * Returns 0 on ok and non-zero on error/failure.
> + */
> +int seccomp_test_filters(int syscall)
> +{
> + int ret = -EACCES;
> + struct seccomp_filter *filter;
> + struct seccomp_filter_metadata metadata;
> +
> + filter = current->seccomp.filter; /* uses task ref */
> + if (!filter)
> + goto out;

Actually, shouldn't having no filter with SECCOMP_MODE_FILTER set be impossible?
It seems better to assure that instead of needlessly testing for it every time.

> +
> + metadata.data.syscall_nr = syscall;
> + metadata.has_args = false;
> +
> +#ifdef CONFIG_COMPAT
> + if (filter->flags.compat != !!(is_compat_task()))
> + goto out;
> +#endif
> +
> + /* Only allow a system call if it is allowed in all ancestors. */
> + ret = 0;
> + for ( ; filter != NULL; filter = filter->parent) {
> + /* Allowed if return value is SECCOMP_BPF_ALLOW */
> + if (seccomp_run_filter(&metadata, sizeof(metadata.data),
> + filter->insns) != SECCOMP_BPF_ALLOW)
> + ret = -EACCES;

Why keep checking filters if one of them fails? Just return -EACCES?

> + }
> +out:
> + return ret;
> +}
> +
> +/**
> + * seccomp_attach_filter: Attaches a seccomp filter to current.
> + * @fprog: BPF program to install
> + *
> + * Context: User context only. This function may sleep on allocation and
> + * operates on current. current must be attempting a system call
> + * when this is called (usually prctl).
> + *
> + * This function may be called repeatedly to install additional filters.
> + * Every filter successfully installed will be evaluated (in reverse order)
> + * for each system call the thread makes.
> + *
> + * Returns 0 on success or an errno on failure.
> + */
> +long seccomp_attach_filter(struct sock_fprog *fprog)
> +{
> + struct seccomp_filter *filter = NULL;
> + /* Note, len is a short so overflow should be impossible. */
> + unsigned long fp_size = fprog->len * sizeof(struct sock_filter);

That's a crude way of doing it. The max is BPF_MAXINSNS and that is checked a
bit late by sk_chk_filter(). But perhaps while you're consolidating the code
with the networking filter code, you could change their code to check that
first before trying to allocate the given amount.

> + long ret = -EPERM;
> +
> + /* Allocate a new seccomp_filter */
> + filter = seccomp_filter_alloc(fp_size);

Actually, your seccomp_filter_alloc() checks for BPF_MAXINSNS already, and if
you change that one as I recommended and you pass on fprog->len directly, no
overflow can happen.

> + if (IS_ERR(filter)) {
> + ret = PTR_ERR(filter);
> + goto out;
> + }
> +
> + /* Copy the instructions from fprog. */
> + ret = -EFAULT;
> + if (copy_from_user(filter->insns, fprog->filter, fp_size))
> + goto out;
> +
> + /* Check the fprog */
> + ret = sk_chk_filter(filter->insns, filter->count);
> + if (ret)
> + goto out;
> +
> + /*
> + * If a process lacks CAP_SYS_ADMIN in its namespace, force
> + * this process and all descendents to run with no_new_privs.
> + * A privileged process will need to set this bit independently,
> + * if desired.
> + */
> + if (security_capable_noaudit(current_cred(), current_user_ns(),
> + CAP_SYS_ADMIN) != 0)
> + current->no_new_privs = 1;
> +
> + /*
> + * If there is an existing filter, make it the parent
> + * and reuse the existing task-based ref.

You're not reusing the existing task-based ref, are you?

You can't reuse it anyway. Perhaps and old leftover comment?

> + */
> + filter->parent = current->seccomp.filter;

I think this needs a more verbose comment, here or elsewhere explaining how
important it is that new filters are added to the start and old filters are
never removed, or else list corruption can happen. This way assures that
forward pointers are always valid, and that's the reason to use a singly
linked list instead of a standard doubly linked list.

> +#ifdef CONFIG_COMPAT
> + /* Disallow changing system calling conventions after the fact. */
> + filter->flags.compat = !!(is_compat_task());
> +
> + if (filter->parent &&
> + filter->parent->flags.compat != filter->flags.compat)
> + return -EACCES;

This shouldn't be possible and checking it here is too late if it somehow
happened anyway. (And you're leaking memory because current->seccomp.filter
isn't updated yet.)

> +#endif
> +
> + /*
> + * Double claim the new filter so we can release it below simplifying
> + * the error paths earlier.
> + */

No it doesn't.

> + ret = 0;
> + get_seccomp_filter(filter);
> + current->seccomp.filter = filter;
> + /* Engage seccomp if it wasn't. This doesn't use PR_SET_SECCOMP. */
> + if (current->seccomp.mode == SECCOMP_MODE_DISABLED) {
> + current->seccomp.mode = SECCOMP_MODE_FILTER;
> + set_thread_flag(TIF_SECCOMP);
> + }
> +

Just add a return 0; here, much simpler and removes 6 lines.

> +out:
> + put_seccomp_filter(filter); /* for get or task, on err */
> + return ret;
> +}
> +
> +#ifdef CONFIG_COMPAT
> +/* This should be kept in sync with net/compat.c which changes infrequently. */
> +struct compat_sock_fprog {
> + u16 len;
> + compat_uptr_t filter; /* struct sock_filter */
> +};
> +
> +static long compat_attach_seccomp_filter(char __user *optval)
> +{
> + struct compat_sock_fprog __user *fprog32 =
> + (struct compat_sock_fprog __user *)optval;
> + struct sock_fprog __user *kfprog =
> + compat_alloc_user_space(sizeof(struct sock_fprog));
> + compat_uptr_t ptr;
> + u16 len;
> +
> + if (!access_ok(VERIFY_READ, fprog32, sizeof(*fprog32)) ||
> + !access_ok(VERIFY_WRITE, kfprog, sizeof(struct sock_fprog)) ||
> + __get_user(len, &fprog32->len) ||
> + __get_user(ptr, &fprog32->filter) ||
> + __put_user(len, &kfprog->len) ||
> + __put_user(compat_ptr(ptr), &kfprog->filter))
> + return -EFAULT;
> +
> + return seccomp_attach_filter(kfprog);
> +}
> +#endif
> +
> +long prctl_attach_seccomp_filter(char __user *user_filter)
> +{
> + struct sock_fprog fprog;
> + long ret = -EINVAL;
> + ret = -EFAULT;

Why not set it to -EFAULT directly?

> + if (!user_filter)
> + goto out;
> +
> +#ifdef CONFIG_COMPAT
> + if (is_compat_task())
> + return compat_attach_seccomp_filter(user_filter);
> +#endif
> +
> + if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
> + goto out;
> +
> + ret = seccomp_attach_filter(&fprog);
> +out:
> + return ret;
> +}
> +
> +/**
> + * seccomp_struct_fork: manages inheritance on fork
> + * @child: forkee's seccomp_struct
> + * @parent: forker's seccomp_struct
> + *
> + * Ensures that @child inherits seccomp mode and state iff

'if'

> + * seccomp filtering is in use.
> + */
> +void seccomp_struct_fork(struct seccomp_struct *child,
> + const struct seccomp_struct *parent)
> +{
> + child->mode = parent->mode;
> + if (parent->mode != SECCOMP_MODE_FILTER)
> + return;
> + child->filter = get_seccomp_filter(parent->filter);

I think how you manage the life times of the filters is very clever and that it
needs some commenting because it is more subtle than it seems at first glance.
The rules seem to be:

- The list is sorted from new to old.

- Never remove old filters.

- The life-time of a group of filters (all filters set by one process between
fork calls) is managed by the one at the head of the group.

This forms a tree with the head filters at forking points with the filters
freed from the leave direction.

> +}
> +
> +/**
> + * seccomp_run_filter - evaluate BPF
> + * @buf: opaque buffer to execute the filter over
> + * @buflen: length of the buffer

Out of date.

> + * @fentry: filter to apply
> + *
> + * Decode and apply filter instructions to the buffer. Return length to
> + * keep, 0 for none.

This doesn't make sense in the context of system calls. Just return 0 for
allowing the system call and -1 to deny it.

> @buf is a seccomp_filter_metadata we are filtering,
> + * @filter is the array of filter instructions. Because all jumps are
> + * guaranteed to be before last instruction, and last instruction
> + * guaranteed to be a RET, we dont need to check flen.
> + *
> + * See core/net/filter.c as this is nearly an exact copy.
> + * At some point, it would be nice to merge them to take advantage of
> + * optimizations (like JIT).
> + */
> +static unsigned int seccomp_run_filter(void *data, uint32_t datalen,
> + const struct sock_filter *fentry)
> +{
> + const void *ptr;
> + u32 A = 0; /* Accumulator */
> + u32 X = 0; /* Index Register */
> + u32 mem[BPF_MEMWORDS]; /* Scratch Memory Store */
> + u32 tmp;
> + int k;
> +
> + /*
> + * Process array of filter instructions.
> + */
> + for (;; fentry++) {
> +#if defined(CONFIG_X86_32)
> +#define K (fentry->k)
> +#else
> + const u32 K = fentry->k;
> +#endif
> +
> + switch (fentry->code) {
> + case BPF_S_ALU_ADD_X:
> + A += X;
> + continue;
> + case BPF_S_ALU_ADD_K:
> + A += K;
> + continue;
> + case BPF_S_ALU_SUB_X:
> + A -= X;
> + continue;
> + case BPF_S_ALU_SUB_K:
> + A -= K;
> + continue;
> + case BPF_S_ALU_MUL_X:
> + A *= X;
> + continue;
> + case BPF_S_ALU_MUL_K:
> + A *= K;
> + continue;
> + case BPF_S_ALU_DIV_X:
> + if (X == 0)
> + return 0;
> + A /= X;
> + continue;
> + case BPF_S_ALU_DIV_K:
> + A = reciprocal_divide(A, K);
> + continue;
> + case BPF_S_ALU_AND_X:
> + A &= X;
> + continue;
> + case BPF_S_ALU_AND_K:
> + A &= K;
> + continue;
> + case BPF_S_ALU_OR_X:
> + A |= X;
> + continue;
> + case BPF_S_ALU_OR_K:
> + A |= K;
> + continue;
> + case BPF_S_ALU_LSH_X:
> + A <<= X;
> + continue;
> + case BPF_S_ALU_LSH_K:
> + A <<= K;
> + continue;
> + case BPF_S_ALU_RSH_X:
> + A >>= X;
> + continue;
> + case BPF_S_ALU_RSH_K:
> + A >>= K;
> + continue;
> + case BPF_S_ALU_NEG:
> + A = -A;
> + continue;
> + case BPF_S_JMP_JA:
> + fentry += K;
> + continue;
> + case BPF_S_JMP_JGT_K:
> + fentry += (A > K) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JGE_K:
> + fentry += (A >= K) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JEQ_K:
> + fentry += (A == K) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JSET_K:
> + fentry += (A & K) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JGT_X:
> + fentry += (A > X) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JGE_X:
> + fentry += (A >= X) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JEQ_X:
> + fentry += (A == X) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_JMP_JSET_X:
> + fentry += (A & X) ? fentry->jt : fentry->jf;
> + continue;
> + case BPF_S_LD_W_ABS:
> + k = K;
> +load_w:
> + ptr = seccomp_load_pointer(data, k, 4, &tmp);
> + if (ptr != NULL) {
> + /*
> + * Assume load_pointer did any byte swapping.
> + */
> + A = *(const u32 *)ptr;
> + continue;
> + }
> + return 0;
> + case BPF_S_LD_H_ABS:
> + k = K;
> +load_h:
> + ptr = seccomp_load_pointer(data, k, 2, &tmp);
> + if (ptr != NULL) {
> + A = *(const u16 *)ptr;
> + continue;
> + }
> + return 0;
> + case BPF_S_LD_B_ABS:
> + k = K;
> +load_b:
> + ptr = seccomp_load_pointer(data, k, 1, &tmp);
> + if (ptr != NULL) {
> + A = *(const u8 *)ptr;
> + continue;
> + }
> + return 0;
> + case BPF_S_LD_W_LEN:
> + A = datalen;
> + continue;
> + case BPF_S_LDX_W_LEN:
> + X = datalen;
> + continue;
> + case BPF_S_LD_W_IND:
> + k = X + K;
> + goto load_w;
> + case BPF_S_LD_H_IND:
> + k = X + K;
> + goto load_h;
> + case BPF_S_LD_B_IND:
> + k = X + K;
> + goto load_b;
> + case BPF_S_LDX_B_MSH:
> + ptr = seccomp_load_pointer(data, K, 1, &tmp);
> + if (ptr != NULL) {
> + X = (*(u8 *)ptr & 0xf) << 2;
> + continue;
> + }
> + return 0;
> + case BPF_S_LD_IMM:
> + A = K;
> + continue;
> + case BPF_S_LDX_IMM:
> + X = K;
> + continue;
> + case BPF_S_LD_MEM:
> + A = mem[K];
> + continue;
> + case BPF_S_LDX_MEM:
> + X = mem[K];
> + continue;
> + case BPF_S_MISC_TAX:
> + X = A;
> + continue;
> + case BPF_S_MISC_TXA:
> + A = X;
> + continue;
> + case BPF_S_RET_K:
> + return K;
> + case BPF_S_RET_A:
> + return A;
> + case BPF_S_ST:
> + mem[K] = A;
> + continue;
> + case BPF_S_STX:
> + mem[K] = X;
> + continue;
> + case BPF_S_ANC_PROTOCOL:
> + case BPF_S_ANC_PKTTYPE:
> + case BPF_S_ANC_IFINDEX:
> + case BPF_S_ANC_MARK:
> + case BPF_S_ANC_QUEUE:
> + case BPF_S_ANC_HATYPE:
> + case BPF_S_ANC_RXHASH:
> + case BPF_S_ANC_CPU:
> + case BPF_S_ANC_NLATTR:
> + case BPF_S_ANC_NLATTR_NEST:
> + continue;
> + default:
> + WARN_RATELIMIT(1, "Unknown code:%u jt:%u tf:%u k:%u\n",
> + fentry->code, fentry->jt,
> + fentry->jf, fentry->k);
> + return 0;
> + }
> + }
> +
> + return 0;
> +}

All in all it seems fairly easy to consolidate this with the networking code.
The only big change would be that the load_pointer() function pointer needs to
be passed, that way the same code can be used for both versions. The filter
checking code needs be expanded to check for network-only ops and reject the
filter if it's a seccomp one.

> diff --git a/kernel/sys.c b/kernel/sys.c
> index 4070153..8e43f70 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1901,6 +1901,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned
> long, arg3,
> case PR_SET_SECCOMP:
> error = prctl_set_seccomp(arg2);
> break;
> + case PR_ATTACH_SECCOMP_FILTER:
> + error = prctl_attach_seccomp_filter((char __user *)
> + arg2);

Please one line.

Actually, wouldn't it make more sense to use PR_SET_SECCOMP and check arg2 to
see if it's filtering, and then get the filter itself from arg3 or something?

Or just stop calling it seccomp of course. PR_ATTACH_SYSCALL_FILTER?

> + break;
> case PR_GET_TSC:
> error = GET_TSC_CTL(arg2);
> break;
> diff --git a/security/Kconfig b/security/Kconfig
> index 51bd5a0..e1ffed8 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -84,6 +84,26 @@ config SECURITY_DMESG_RESTRICT
>
> If you are unsure how to answer this question, answer N.
>
> +config SECCOMP_FILTER
> + bool "Enable seccomp-based system call filtering"
> + select SECCOMP
> + help
> + This option provide support for limiting the accessibility of
> + systems calls at a task-level using a dynamically defined policy.
> +
> + System call filtering policy is expressed by the user using
> + a Berkeley Packet Filter program. The program is attached using
> + prctl(2). For every system call the task makes, its number,
> + arguments, and other metadata will be evaluated by the attached
> + filter program. The result determines if the system call may
> + may proceed or if the task should be terminated.
> +
> + This behavior is meant to aid security-conscious software in
> + its ability to minimize the risk of running potentially
> + risky code.
> +
> + See Documentation/prctl/seccomp_filter.txt for more detail.
> +
> config SECURITY
> bool "Enable different security models"
> depends on SYSFS
> --
> 1.7.5.4
>
>

Greetings,

Indan




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