lkml.org 
[lkml]   [2012]   [Apr]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH v18 15/15] Documentation: prctl/seccomp_filter
On Thu, Apr 12, 2012 at 5:48 PM, Will Drewry <wad@chromium.org> wrote:
> Documents how system call filtering using Berkeley Packet
> Filter programs works and how it may be used.
> Includes an example for x86 and a semi-generic
> example using a macro-based code generator.

Hi guys, that is quite the To/CC list. I didn't have the heart to add
yet another address (linux-next) to it...

Can you have a look at this link? It appears to be breaking
the i386 allmodconfig builds in linux-next.

http://kisskb.ellerman.id.au/kisskb/buildresult/6123842/

"samples/seccomp/bpf-direct.c:47:2: error: #error Unsupported platform"

Thanks,
Paul.
--

>
> Acked-by: Eric Paris <eparis@redhat.com>
> Signed-off-by: Will Drewry <wad@chromium.org>
>
> v18: - added acked by
>     - update no new privs numbers
> v17: - remove @compat note and add Pitfalls section for arch checking
>       (keescook@chromium.org)
> v16: -
> v15: -
> v14: - rebase/nochanges
> v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
> v12: - comment on the ptrace_event use
>     - update arch support comment
>     - note the behavior of SECCOMP_RET_DATA when there are multiple filters
>       (keescook@chromium.org)
>     - lots of samples/ clean up incl 64-bit bpf-direct support
>       (markus@chromium.org)
>     - rebase to linux-next
> v11: - overhaul return value language, updates (keescook@chromium.org)
>     - comment on do_exit(SIGSYS)
> v10: - update for SIGSYS
>     - update for new seccomp_data layout
>     - update for ptrace option use
> v9: - updated bpf-direct.c for SIGILL
> v8: - add PR_SET_NO_NEW_PRIVS to the samples.
> v7: - updated for all the new stuff in v7: TRAP, TRACE
>    - only talk about PR_SET_SECCOMP now
>    - fixed bad JLE32 check (coreyb@linux.vnet.ibm.com)
>    - adds dropper.c: a simple system call disabler
> v6: - tweak the language to note the requirement of
>      PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu)
> v5: - update sample to use system call arguments
>    - adds a "fancy" example using a macro-based generator
>    - cleaned up bpf in the sample
>    - update docs to mention arguments
>    - fix prctl value (eparis@redhat.com)
>    - language cleanup (rdunlap@xenotime.net)
> v4: - update for no_new_privs use
>    - minor tweaks
> v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net)
>    - document use of tentative always-unprivileged
>    - guard sample compilation for i386 and x86_64
> v2: - move code to samples (corbet@lwn.net)
> ---
>  Documentation/prctl/seccomp_filter.txt |  163 ++++++++++++++++++++++
>  samples/Makefile                       |    2 +-
>  samples/seccomp/Makefile               |   38 +++++
>  samples/seccomp/bpf-direct.c           |  176 +++++++++++++++++++++++
>  samples/seccomp/bpf-fancy.c            |  102 ++++++++++++++
>  samples/seccomp/bpf-helper.c           |   89 ++++++++++++
>  samples/seccomp/bpf-helper.h           |  238 ++++++++++++++++++++++++++++++++
>  samples/seccomp/dropper.c              |   68 +++++++++
>  8 files changed, 875 insertions(+), 1 deletions(-)
>  create mode 100644 Documentation/prctl/seccomp_filter.txt
>  create mode 100644 samples/seccomp/Makefile
>  create mode 100644 samples/seccomp/bpf-direct.c
>  create mode 100644 samples/seccomp/bpf-fancy.c
>  create mode 100644 samples/seccomp/bpf-helper.c
>  create mode 100644 samples/seccomp/bpf-helper.h
>  create mode 100644 samples/seccomp/dropper.c
>
> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
> new file mode 100644
> index 0000000..597c3c5
> --- /dev/null
> +++ b/Documentation/prctl/seccomp_filter.txt
> @@ -0,0 +1,163 @@
> +               SECure COMPuting with filters
> +               =============================
> +
> +Introduction
> +------------
> +
> +A large number of system calls are exposed to every userland process
> +with many of them going unused for the entire lifetime of the process.
> +As system calls change and mature, bugs are found and eradicated.  A
> +certain subset of userland applications benefit by having a reduced set
> +of available system calls.  The resulting set reduces the total kernel
> +surface exposed to the application.  System call filtering is meant for
> +use with those applications.
> +
> +Seccomp filtering provides a means for a process to specify a filter for
> +incoming system calls.  The filter is expressed as a Berkeley Packet
> +Filter (BPF) program, as with socket filters, except that the data
> +operated on is related to the system call being made: system call
> +number and the system call arguments.  This allows for expressive
> +filtering of system calls using a filter program language with a long
> +history of being exposed to userland and a straightforward data set.
> +
> +Additionally, BPF makes it impossible for users of seccomp to fall prey
> +to time-of-check-time-of-use (TOCTOU) attacks that are common in system
> +call interposition frameworks.  BPF programs may not dereference
> +pointers which constrains all filters to solely evaluating the system
> +call arguments directly.
> +
> +What it isn't
> +-------------
> +
> +System call filtering isn't a sandbox.  It provides a clearly defined
> +mechanism for minimizing the exposed kernel surface.  It is meant to be
> +a tool for sandbox developers to use.  Beyond that, policy for logical
> +behavior and information flow should be managed with a combination of
> +other system hardening techniques and, potentially, an LSM of your
> +choosing.  Expressive, dynamic filters provide further options down this
> +path (avoiding pathological sizes or selecting which of the multiplexed
> +system calls in socketcall() is allowed, for instance) which could be
> +construed, incorrectly, as a more complete sandboxing solution.
> +
> +Usage
> +-----
> +
> +An additional seccomp mode is added and is enabled using the same
> +prctl(2) call as the strict seccomp.  If the architecture has
> +CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below:
> +
> +PR_SET_SECCOMP:
> +       Now takes an additional argument which specifies a new filter
> +       using a BPF program.
> +       The BPF program will be executed over struct seccomp_data
> +       reflecting the system call number, arguments, and other
> +       metadata.  The BPF program must then return one of the
> +       acceptable values to inform the kernel which action should be
> +       taken.
> +
> +       Usage:
> +               prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog);
> +
> +       The 'prog' argument is a pointer to a struct sock_fprog which
> +       will contain the filter program.  If the program is invalid, the
> +       call will return -1 and set errno to EINVAL.
> +
> +       If fork/clone and execve are allowed by @prog, any child
> +       processes will be constrained to the same filters and system
> +       call ABI as the parent.
> +
> +       Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or
> +       run with CAP_SYS_ADMIN privileges in its namespace.  If these are not
> +       true, -EACCES will be returned.  This requirement ensures that filter
> +       programs cannot be applied to child processes with greater privileges
> +       than the task that installed them.
> +
> +       Additionally, if prctl(2) is allowed by the attached filter,
> +       additional filters may be layered on which will increase evaluation
> +       time, but allow for further decreasing the attack surface during
> +       execution of a process.
> +
> +The above call returns 0 on success and non-zero on error.
> +
> +Return values
> +-------------
> +A seccomp filter may return any of the following values. If multiple
> +filters exist, the return value for the evaluation of a given system
> +call will always use the highest precedent value. (For example,
> +SECCOMP_RET_KILL will always take precedence.)
> +
> +In precedence order, they are:
> +
> +SECCOMP_RET_KILL:
> +       Results in the task exiting immediately without executing the
> +       system call.  The exit status of the task (status & 0x7f) will
> +       be SIGSYS, not SIGKILL.
> +
> +SECCOMP_RET_TRAP:
> +       Results in the kernel sending a SIGSYS signal to the triggering
> +       task without executing the system call.  The kernel will
> +       rollback the register state to just before the system call
> +       entry such that a signal handler in the task will be able to
> +       inspect the ucontext_t->uc_mcontext registers and emulate
> +       system call success or failure upon return from the signal
> +       handler.
> +
> +       The SECCOMP_RET_DATA portion of the return value will be passed
> +       as si_errno.
> +
> +       SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP.
> +
> +SECCOMP_RET_ERRNO:
> +       Results in the lower 16-bits of the return value being passed
> +       to userland as the errno without executing the system call.
> +
> +SECCOMP_RET_TRACE:
> +       When returned, this value will cause the kernel to attempt to
> +       notify a ptrace()-based tracer prior to executing the system
> +       call.  If there is no tracer present, -ENOSYS is returned to
> +       userland and the system call is not executed.
> +
> +       A tracer will be notified if it requests PTRACE_O_TRACESECCOMP
> +       using ptrace(PTRACE_SETOPTIONS).  The tracer will be notified
> +       of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of
> +       the BPF program return value will be available to the tracer
> +       via PTRACE_GETEVENTMSG.
> +
> +SECCOMP_RET_ALLOW:
> +       Results in the system call being executed.
> +
> +If multiple filters exist, the return value for the evaluation of a
> +given system call will always use the highest precedent value.
> +
> +Precedence is only determined using the SECCOMP_RET_ACTION mask.  When
> +multiple filters return values of the same precedence, only the
> +SECCOMP_RET_DATA from the most recently installed filter will be
> +returned.
> +
> +Pitfalls
> +--------
> +
> +The biggest pitfall to avoid during use is filtering on system call
> +number without checking the architecture value.  Why?  On any
> +architecture that supports multiple system call invocation conventions,
> +the system call numbers may vary based on the specific invocation.  If
> +the numbers in the different calling conventions overlap, then checks in
> +the filters may be abused.  Always check the arch value!
> +
> +Example
> +-------
> +
> +The samples/seccomp/ directory contains both an x86-specific example
> +and a more generic example of a higher level macro interface for BPF
> +program generation.
> +
> +
> +
> +Adding architecture support
> +-----------------------
> +
> +See arch/Kconfig for the authoritative requirements.  In general, if an
> +architecture supports both ptrace_event and seccomp, it will be able to
> +support seccomp filter with minor fixup: SIGSYS support and seccomp return
> +value checking.  Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
> +to its arch-specific Kconfig.
> diff --git a/samples/Makefile b/samples/Makefile
> index 2f75851..5ef08bb 100644
> --- a/samples/Makefile
> +++ b/samples/Makefile
> @@ -1,4 +1,4 @@
>  # Makefile for Linux samples code
>
>  obj-$(CONFIG_SAMPLES)  += kobject/ kprobes/ tracepoints/ trace_events/ \
> -                          hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/
> +                          hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/
> diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
> new file mode 100644
> index 0000000..e8fe0f5
> --- /dev/null
> +++ b/samples/seccomp/Makefile
> @@ -0,0 +1,38 @@
> +# kbuild trick to avoid linker error. Can be omitted if a module is built.
> +obj- := dummy.o
> +
> +hostprogs-$(CONFIG_SECCOMP) := bpf-fancy dropper
> +bpf-fancy-objs := bpf-fancy.o bpf-helper.o
> +
> +HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
> +HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
> +HOSTCFLAGS_bpf-helper.o += -I$(objtree)/usr/include
> +HOSTCFLAGS_bpf-helper.o += -idirafter $(objtree)/include
> +
> +HOSTCFLAGS_dropper.o += -I$(objtree)/usr/include
> +HOSTCFLAGS_dropper.o += -idirafter $(objtree)/include
> +dropper-objs := dropper.o
> +
> +# bpf-direct.c is x86-only.
> +ifeq ($(SRCARCH),x86)
> +# List of programs to build
> +hostprogs-$(CONFIG_SECCOMP) += bpf-direct
> +bpf-direct-objs := bpf-direct.o
> +endif
> +
> +HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include
> +HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
> +
> +# Try to match the kernel target.
> +ifeq ($(CONFIG_64BIT),)
> +HOSTCFLAGS_bpf-direct.o += -m32
> +HOSTCFLAGS_dropper.o += -m32
> +HOSTCFLAGS_bpf-helper.o += -m32
> +HOSTCFLAGS_bpf-fancy.o += -m32
> +HOSTLOADLIBES_bpf-direct += -m32
> +HOSTLOADLIBES_bpf-fancy += -m32
> +HOSTLOADLIBES_dropper += -m32
> +endif
> +
> +# Tell kbuild to always build the programs
> +always := $(hostprogs-y)
> diff --git a/samples/seccomp/bpf-direct.c b/samples/seccomp/bpf-direct.c
> new file mode 100644
> index 0000000..26f523e
> --- /dev/null
> +++ b/samples/seccomp/bpf-direct.c
> @@ -0,0 +1,176 @@
> +/*
> + * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
> + * Author: Will Drewry <wad@chromium.org>
> + *
> + * The code may be used by anyone for any purpose,
> + * and can serve as a starting point for developing
> + * applications using prctl(PR_SET_SECCOMP, 2, ...).
> + */
> +#define __USE_GNU 1
> +#define _GNU_SOURCE 1
> +
> +#include <linux/types.h>
> +#include <linux/filter.h>
> +#include <linux/seccomp.h>
> +#include <linux/unistd.h>
> +#include <signal.h>
> +#include <stdio.h>
> +#include <stddef.h>
> +#include <string.h>
> +#include <sys/prctl.h>
> +#include <unistd.h>
> +
> +#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
> +#define syscall_nr (offsetof(struct seccomp_data, nr))
> +
> +#if defined(__i386__)
> +#define REG_RESULT     REG_EAX
> +#define REG_SYSCALL    REG_EAX
> +#define REG_ARG0       REG_EBX
> +#define REG_ARG1       REG_ECX
> +#define REG_ARG2       REG_EDX
> +#define REG_ARG3       REG_ESI
> +#define REG_ARG4       REG_EDI
> +#define REG_ARG5       REG_EBP
> +#elif defined(__x86_64__)
> +#define REG_RESULT     REG_RAX
> +#define REG_SYSCALL    REG_RAX
> +#define REG_ARG0       REG_RDI
> +#define REG_ARG1       REG_RSI
> +#define REG_ARG2       REG_RDX
> +#define REG_ARG3       REG_R10
> +#define REG_ARG4       REG_R8
> +#define REG_ARG5       REG_R9
> +#else
> +#error Unsupported platform
> +#endif
> +
> +#ifndef PR_SET_NO_NEW_PRIVS
> +#define PR_SET_NO_NEW_PRIVS 38
> +#endif
> +
> +#ifndef SYS_SECCOMP
> +#define SYS_SECCOMP 1
> +#endif
> +
> +static void emulator(int nr, siginfo_t *info, void *void_context)
> +{
> +       ucontext_t *ctx = (ucontext_t *)(void_context);
> +       int syscall;
> +       char *buf;
> +       ssize_t bytes;
> +       size_t len;
> +       if (info->si_code != SYS_SECCOMP)
> +               return;
> +       if (!ctx)
> +               return;
> +       syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
> +       buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
> +       len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
> +
> +       if (syscall != __NR_write)
> +               return;
> +       if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
> +               return;
> +       /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
> +       ctx->uc_mcontext.gregs[REG_RESULT] = -1;
> +       if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
> +               bytes = write(STDOUT_FILENO, buf, len);
> +               ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
> +       }
> +       return;
> +}
> +
> +static int install_emulator(void)
> +{
> +       struct sigaction act;
> +       sigset_t mask;
> +       memset(&act, 0, sizeof(act));
> +       sigemptyset(&mask);
> +       sigaddset(&mask, SIGSYS);
> +
> +       act.sa_sigaction = &emulator;
> +       act.sa_flags = SA_SIGINFO;
> +       if (sigaction(SIGSYS, &act, NULL) < 0) {
> +               perror("sigaction");
> +               return -1;
> +       }
> +       if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
> +               perror("sigprocmask");
> +               return -1;
> +       }
> +       return 0;
> +}
> +
> +static int install_filter(void)
> +{
> +       struct sock_filter filter[] = {
> +               /* Grab the system call number */
> +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
> +               /* Jump table for the allowed syscalls */
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +#ifdef __NR_sigreturn
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +#endif
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
> +
> +               /* Check that read is only using stdin. */
> +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
> +
> +               /* Check that write is only using stdout */
> +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
> +               /* Trap attempts to write to stderr */
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
> +
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
> +       };
> +       struct sock_fprog prog = {
> +               .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
> +               .filter = filter,
> +       };
> +
> +       if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
> +               perror("prctl(NO_NEW_PRIVS)");
> +               return 1;
> +       }
> +
> +
> +       if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
> +               perror("prctl");
> +               return 1;
> +       }
> +       return 0;
> +}
> +
> +#define payload(_c) (_c), sizeof((_c))
> +int main(int argc, char **argv)
> +{
> +       char buf[4096];
> +       ssize_t bytes = 0;
> +       if (install_emulator())
> +               return 1;
> +       if (install_filter())
> +               return 1;
> +       syscall(__NR_write, STDOUT_FILENO,
> +               payload("OHAI! WHAT IS YOUR NAME? "));
> +       bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
> +       syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
> +       syscall(__NR_write, STDOUT_FILENO, buf, bytes);
> +       syscall(__NR_write, STDERR_FILENO,
> +               payload("Error message going to STDERR\n"));
> +       return 0;
> +}
> diff --git a/samples/seccomp/bpf-fancy.c b/samples/seccomp/bpf-fancy.c
> new file mode 100644
> index 0000000..8eb483a
> --- /dev/null
> +++ b/samples/seccomp/bpf-fancy.c
> @@ -0,0 +1,102 @@
> +/*
> + * Seccomp BPF example using a macro-based generator.
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
> + * Author: Will Drewry <wad@chromium.org>
> + *
> + * The code may be used by anyone for any purpose,
> + * and can serve as a starting point for developing
> + * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
> + */
> +
> +#include <linux/filter.h>
> +#include <linux/seccomp.h>
> +#include <linux/unistd.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <sys/prctl.h>
> +#include <unistd.h>
> +
> +#include "bpf-helper.h"
> +
> +#ifndef PR_SET_NO_NEW_PRIVS
> +#define PR_SET_NO_NEW_PRIVS 38
> +#endif
> +
> +int main(int argc, char **argv)
> +{
> +       struct bpf_labels l;
> +       static const char msg1[] = "Please type something: ";
> +       static const char msg2[] = "You typed: ";
> +       char buf[256];
> +       struct sock_filter filter[] = {
> +               /* TODO: LOAD_SYSCALL_NR(arch) and enforce an arch */
> +               LOAD_SYSCALL_NR,
> +               SYSCALL(__NR_exit, ALLOW),
> +               SYSCALL(__NR_exit_group, ALLOW),
> +               SYSCALL(__NR_write, JUMP(&l, write_fd)),
> +               SYSCALL(__NR_read, JUMP(&l, read)),
> +               DENY,  /* Don't passthrough into a label */
> +
> +               LABEL(&l, read),
> +               ARG(0),
> +               JNE(STDIN_FILENO, DENY),
> +               ARG(1),
> +               JNE((unsigned long)buf, DENY),
> +               ARG(2),
> +               JGE(sizeof(buf), DENY),
> +               ALLOW,
> +
> +               LABEL(&l, write_fd),
> +               ARG(0),
> +               JEQ(STDOUT_FILENO, JUMP(&l, write_buf)),
> +               JEQ(STDERR_FILENO, JUMP(&l, write_buf)),
> +               DENY,
> +
> +               LABEL(&l, write_buf),
> +               ARG(1),
> +               JEQ((unsigned long)msg1, JUMP(&l, msg1_len)),
> +               JEQ((unsigned long)msg2, JUMP(&l, msg2_len)),
> +               JEQ((unsigned long)buf, JUMP(&l, buf_len)),
> +               DENY,
> +
> +               LABEL(&l, msg1_len),
> +               ARG(2),
> +               JLT(sizeof(msg1), ALLOW),
> +               DENY,
> +
> +               LABEL(&l, msg2_len),
> +               ARG(2),
> +               JLT(sizeof(msg2), ALLOW),
> +               DENY,
> +
> +               LABEL(&l, buf_len),
> +               ARG(2),
> +               JLT(sizeof(buf), ALLOW),
> +               DENY,
> +       };
> +       struct sock_fprog prog = {
> +               .filter = filter,
> +               .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
> +       };
> +       ssize_t bytes;
> +       bpf_resolve_jumps(&l, filter, sizeof(filter)/sizeof(*filter));
> +
> +       if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
> +               perror("prctl(NO_NEW_PRIVS)");
> +               return 1;
> +       }
> +
> +       if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
> +               perror("prctl(SECCOMP)");
> +               return 1;
> +       }
> +       syscall(__NR_write, STDOUT_FILENO, msg1, strlen(msg1));
> +       bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)-1);
> +       bytes = (bytes > 0 ? bytes : 0);
> +       syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2));
> +       syscall(__NR_write, STDERR_FILENO, buf, bytes);
> +       /* Now get killed */
> +       syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2)+2);
> +       return 0;
> +}
> diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c
> new file mode 100644
> index 0000000..579cfe3
> --- /dev/null
> +++ b/samples/seccomp/bpf-helper.c
> @@ -0,0 +1,89 @@
> +/*
> + * Seccomp BPF helper functions
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
> + * Author: Will Drewry <wad@chromium.org>
> + *
> + * The code may be used by anyone for any purpose,
> + * and can serve as a starting point for developing
> + * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
> + */
> +
> +#include <stdio.h>
> +#include <string.h>
> +
> +#include "bpf-helper.h"
> +
> +int bpf_resolve_jumps(struct bpf_labels *labels,
> +                     struct sock_filter *filter, size_t count)
> +{
> +       struct sock_filter *begin = filter;
> +       __u8 insn = count - 1;
> +
> +       if (count < 1)
> +               return -1;
> +       /*
> +       * Walk it once, backwards, to build the label table and do fixups.
> +       * Since backward jumps are disallowed by BPF, this is easy.
> +       */
> +       filter += insn;
> +       for (; filter >= begin; --insn, --filter) {
> +               if (filter->code != (BPF_JMP+BPF_JA))
> +                       continue;
> +               switch ((filter->jt<<8)|filter->jf) {
> +               case (JUMP_JT<<8)|JUMP_JF:
> +                       if (labels->labels[filter->k].location == 0xffffffff) {
> +                               fprintf(stderr, "Unresolved label: '%s'\n",
> +                                       labels->labels[filter->k].label);
> +                               return 1;
> +                       }
> +                       filter->k = labels->labels[filter->k].location -
> +                                   (insn + 1);
> +                       filter->jt = 0;
> +                       filter->jf = 0;
> +                       continue;
> +               case (LABEL_JT<<8)|LABEL_JF:
> +                       if (labels->labels[filter->k].location != 0xffffffff) {
> +                               fprintf(stderr, "Duplicate label use: '%s'\n",
> +                                       labels->labels[filter->k].label);
> +                               return 1;
> +                       }
> +                       labels->labels[filter->k].location = insn;
> +                       filter->k = 0; /* fall through */
> +                       filter->jt = 0;
> +                       filter->jf = 0;
> +                       continue;
> +               }
> +       }
> +       return 0;
> +}
> +
> +/* Simple lookup table for labels. */
> +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
> +{
> +       struct __bpf_label *begin = labels->labels, *end;
> +       int id;
> +       if (labels->count == 0) {
> +               begin->label = label;
> +               begin->location = 0xffffffff;
> +               labels->count++;
> +               return 0;
> +       }
> +       end = begin + labels->count;
> +       for (id = 0; begin < end; ++begin, ++id) {
> +               if (!strcmp(label, begin->label))
> +                       return id;
> +       }
> +       begin->label = label;
> +       begin->location = 0xffffffff;
> +       labels->count++;
> +       return id;
> +}
> +
> +void seccomp_bpf_print(struct sock_filter *filter, size_t count)
> +{
> +       struct sock_filter *end = filter + count;
> +       for ( ; filter < end; ++filter)
> +               printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
> +                       filter->code, filter->jt, filter->jf, filter->k);
> +}
> diff --git a/samples/seccomp/bpf-helper.h b/samples/seccomp/bpf-helper.h
> new file mode 100644
> index 0000000..643279d
> --- /dev/null
> +++ b/samples/seccomp/bpf-helper.h
> @@ -0,0 +1,238 @@
> +/*
> + * Example wrapper around BPF macros.
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
> + * Author: Will Drewry <wad@chromium.org>
> + *
> + * The code may be used by anyone for any purpose,
> + * and can serve as a starting point for developing
> + * applications using prctl(PR_SET_SECCOMP, 2, ...).
> + *
> + * No guarantees are provided with respect to the correctness
> + * or functionality of this code.
> + */
> +#ifndef __BPF_HELPER_H__
> +#define __BPF_HELPER_H__
> +
> +#include <asm/bitsperlong.h>   /* for __BITS_PER_LONG */
> +#include <endian.h>
> +#include <linux/filter.h>
> +#include <linux/seccomp.h>     /* for seccomp_data */
> +#include <linux/types.h>
> +#include <linux/unistd.h>
> +#include <stddef.h>
> +
> +#define BPF_LABELS_MAX 256
> +struct bpf_labels {
> +       int count;
> +       struct __bpf_label {
> +               const char *label;
> +               __u32 location;
> +       } labels[BPF_LABELS_MAX];
> +};
> +
> +int bpf_resolve_jumps(struct bpf_labels *labels,
> +                     struct sock_filter *filter, size_t count);
> +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label);
> +void seccomp_bpf_print(struct sock_filter *filter, size_t count);
> +
> +#define JUMP_JT 0xff
> +#define JUMP_JF 0xff
> +#define LABEL_JT 0xfe
> +#define LABEL_JF 0xfe
> +
> +#define ALLOW \
> +       BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
> +#define DENY \
> +       BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
> +#define JUMP(labels, label) \
> +       BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
> +                JUMP_JT, JUMP_JF)
> +#define LABEL(labels, label) \
> +       BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
> +                LABEL_JT, LABEL_JF)
> +#define SYSCALL(nr, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (nr), 0, 1), \
> +       jt
> +
> +/* Lame, but just an example */
> +#define FIND_LABEL(labels, label) seccomp_bpf_label((labels), #label)
> +
> +#define EXPAND(...) __VA_ARGS__
> +/* Map all width-sensitive operations */
> +#if __BITS_PER_LONG == 32
> +
> +#define JEQ(x, jt) JEQ32(x, EXPAND(jt))
> +#define JNE(x, jt) JNE32(x, EXPAND(jt))
> +#define JGT(x, jt) JGT32(x, EXPAND(jt))
> +#define JLT(x, jt) JLT32(x, EXPAND(jt))
> +#define JGE(x, jt) JGE32(x, EXPAND(jt))
> +#define JLE(x, jt) JLE32(x, EXPAND(jt))
> +#define JA(x, jt) JA32(x, EXPAND(jt))
> +#define ARG(i) ARG_32(i)
> +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
> +
> +#elif __BITS_PER_LONG == 64
> +
> +/* Ensure that we load the logically correct offset. */
> +#if __BYTE_ORDER == __LITTLE_ENDIAN
> +#define ENDIAN(_lo, _hi) _lo, _hi
> +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
> +#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
> +#elif __BYTE_ORDER == __BIG_ENDIAN
> +#define ENDIAN(_lo, _hi) _hi, _lo
> +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
> +#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
> +#else
> +#error "Unknown endianness"
> +#endif
> +
> +union arg64 {
> +       struct {
> +               __u32 ENDIAN(lo32, hi32);
> +       };
> +       __u64 u64;
> +};
> +
> +#define JEQ(x, jt) \
> +       JEQ64(((union arg64){.u64 = (x)}).lo32, \
> +             ((union arg64){.u64 = (x)}).hi32, \
> +             EXPAND(jt))
> +#define JGT(x, jt) \
> +       JGT64(((union arg64){.u64 = (x)}).lo32, \
> +             ((union arg64){.u64 = (x)}).hi32, \
> +             EXPAND(jt))
> +#define JGE(x, jt) \
> +       JGE64(((union arg64){.u64 = (x)}).lo32, \
> +             ((union arg64){.u64 = (x)}).hi32, \
> +             EXPAND(jt))
> +#define JNE(x, jt) \
> +       JNE64(((union arg64){.u64 = (x)}).lo32, \
> +             ((union arg64){.u64 = (x)}).hi32, \
> +             EXPAND(jt))
> +#define JLT(x, jt) \
> +       JLT64(((union arg64){.u64 = (x)}).lo32, \
> +             ((union arg64){.u64 = (x)}).hi32, \
> +             EXPAND(jt))
> +#define JLE(x, jt) \
> +       JLE64(((union arg64){.u64 = (x)}).lo32, \
> +             ((union arg64){.u64 = (x)}).hi32, \
> +             EXPAND(jt))
> +
> +#define JA(x, jt) \
> +       JA64(((union arg64){.u64 = (x)}).lo32, \
> +              ((union arg64){.u64 = (x)}).hi32, \
> +              EXPAND(jt))
> +#define ARG(i) ARG_64(i)
> +
> +#else
> +#error __BITS_PER_LONG value unusable.
> +#endif
> +
> +/* Loads the arg into A */
> +#define ARG_32(idx) \
> +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx))
> +
> +/* Loads hi into A and lo in X */
> +#define ARG_64(idx) \
> +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx)), \
> +       BPF_STMT(BPF_ST, 0), /* lo -> M[0] */ \
> +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, HI_ARG(idx)), \
> +       BPF_STMT(BPF_ST, 1) /* hi -> M[1] */
> +
> +#define JEQ32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 0, 1), \
> +       jt
> +
> +#define JNE32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 1, 0), \
> +       jt
> +
> +/* Checks the lo, then swaps to check the hi. A=lo,X=hi */
> +#define JEQ64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 0, 2), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define JNE64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 5, 0), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 2, 0), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define JA32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (value), 0, 1), \
> +       jt
> +
> +#define JA64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (hi), 3, 0), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (lo), 0, 2), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define JGE32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 0, 1), \
> +       jt
> +
> +#define JLT32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 1, 0), \
> +       jt
> +
> +/* Shortcut checking if hi > arg.hi. */
> +#define JGE64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (lo), 0, 2), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define JLT64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (hi), 0, 4), \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define JGT32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 0, 1), \
> +       jt
> +
> +#define JLE32(value, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 1, 0), \
> +       jt
> +
> +/* Check hi > args.hi first, then do the GE checking */
> +#define JGT64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 0, 2), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define JLE64(lo, hi, jt) \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 6, 0), \
> +       BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 3), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
> +       BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \
> +       jt, \
> +       BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */
> +
> +#define LOAD_SYSCALL_NR \
> +       BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
> +                offsetof(struct seccomp_data, nr))
> +
> +#endif  /* __BPF_HELPER_H__ */
> diff --git a/samples/seccomp/dropper.c b/samples/seccomp/dropper.c
> new file mode 100644
> index 0000000..c69c347
> --- /dev/null
> +++ b/samples/seccomp/dropper.c
> @@ -0,0 +1,68 @@
> +/*
> + * Naive system call dropper built on seccomp_filter.
> + *
> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
> + * Author: Will Drewry <wad@chromium.org>
> + *
> + * The code may be used by anyone for any purpose,
> + * and can serve as a starting point for developing
> + * applications using prctl(PR_SET_SECCOMP, 2, ...).
> + *
> + * When run, returns the specified errno for the specified
> + * system call number against the given architecture.
> + *
> + * Run this one as root as PR_SET_NO_NEW_PRIVS is not called.
> + */
> +
> +#include <errno.h>
> +#include <linux/audit.h>
> +#include <linux/filter.h>
> +#include <linux/seccomp.h>
> +#include <linux/unistd.h>
> +#include <stdio.h>
> +#include <stddef.h>
> +#include <stdlib.h>
> +#include <sys/prctl.h>
> +#include <unistd.h>
> +
> +static int install_filter(int nr, int arch, int error)
> +{
> +       struct sock_filter filter[] = {
> +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
> +                        (offsetof(struct seccomp_data, arch))),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
> +               BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
> +                        (offsetof(struct seccomp_data, nr))),
> +               BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
> +               BPF_STMT(BPF_RET+BPF_K,
> +                        SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
> +               BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +       };
> +       struct sock_fprog prog = {
> +               .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
> +               .filter = filter,
> +       };
> +       if (prctl(PR_SET_SECCOMP, 2, &prog)) {
> +               perror("prctl");
> +               return 1;
> +       }
> +       return 0;
> +}
> +
> +int main(int argc, char **argv)
> +{
> +       if (argc < 5) {
> +               fprintf(stderr, "Usage:\n"
> +                       "dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
> +                       "Hint:  AUDIT_ARCH_I386: 0x%X\n"
> +                       "       AUDIT_ARCH_X86_64: 0x%X\n"
> +                       "\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
> +               return 1;
> +       }
> +       if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
> +                          strtol(argv[3], NULL, 0)))
> +               return 1;
> +       execv(argv[4], &argv[4]);
> +       printf("Failed to execv\n");
> +       return 255;
> +}
> --
> 1.7.5.4
>
> --
> To unsubscribe from this list: send the line "unsubscribe netdev" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2012-04-18 22:37    [from the cache]
©2003-2011 Jasper Spaans