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SubjectRe: [rfc 4/4] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation, v3
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On Thu, Jul 24, 2014 at 9:47 AM, Cyrill Gorcunov <gorcunov@openvz.org> wrote:
> During development of c/r we've noticed that in case if we need to
> support user namespaces we face a problem with capabilities in
> prctl(PR_SET_MM, ...) call, in particular once new user namespace
> is created capable(CAP_SYS_RESOURCE) no longer passes.
>
> A approach is to eliminate CAP_SYS_RESOURCE check but pass all
> new values in one bundle, which would allow the kernel to make
> more intensive test for sanity of values and same time allow us to
> support checkpoint/restore of user namespaces.
>
> Thus a new command PR_SET_MM_MAP introduced. It takes a pointer of
> prctl_mm_map structure which carries all the members to be updated.
>
> prctl(PR_SET_MM, PR_SET_MM_MAP, struct prctl_mm_map *, size)
>
> struct prctl_mm_map {
> __u64 start_code;
> __u64 end_code;
> __u64 start_data;
> __u64 end_data;
> __u64 start_brk;
> __u64 brk;
> __u64 start_stack;
> __u64 arg_start;
> __u64 arg_end;
> __u64 env_start;
> __u64 env_end;
> __u64 *auxv;
> __u32 auxv_size;
> __u32 exe_fd;
> };
>
> All members except @exe_fd correspond ones of struct mm_struct.
> To figure out which available values these members may take here
> are meanings of the members.
>
> - start_code, end_code: represent bounds of executable code area
> - start_data, end_data: represent bounds of data area
> - start_brk, brk: used to calculate bounds for brk() syscall
> - start_stack: used when accounting space needed for command
> line arguments, environment and shmat() syscall
> - arg_start, arg_end, env_start, env_end: represent memory area
> supplied for command line arguments and environment variables
> - auxv, auxv_size: carries auxiliary vector, Elf format specifics
> - exe_fd: file descriptor number for executable link (/proc/self/exe)
>
> Thus we apply the following requirements to the values
>
> 1) Any member except @auxv, @auxv_size, @exe_fd is rather an address
> in user space thus it must be laying inside [mmap_min_addr, mmap_max_addr)
> interval.
>
> 2) While @[start|end]_code and @[start|end]_data may point to an nonexisting
> VMAs (say a program maps own new .text and .data segments during execution)
> the rest of members should belong to VMA which must exist.
>
> 3) Addresses must be ordered, ie @start_ member must not be greater or
> equal to appropriate @end_ member.
>
> 4) As in regular Elf loading procedure we require that @start_brk and
> @brk be greater than @end_data.
>
> 5) If RLIMIT_DATA rlimit is set to non-infinity new values should not
> exceed existing limit. Same applies to RLIMIT_STACK.
>
> 6) Auxiliary vector size must not exceed existing one (which is
> predefined as AT_VECTOR_SIZE and depends on architecture).
>
> 7) File descriptor passed in @exe_file should be pointing
> to executable file (because we use existing prctl_set_mm_exe_file_locked
> helper it ensures that the file we are going to use as exe link has all
> required permission granted).
>
> Now about where these members are involved inside kernel code:
>
> - @start_code and @end_code are used in /proc/$pid/[stat|statm] output;
>
> - @start_data and @end_data are used in /proc/$pid/[stat|statm] output,
> also they are considered if there enough space for brk() syscall
> result if RLIMIT_DATA is set;
>
> - @start_brk shown in /proc/$pid/stat output and accounted in brk()
> syscall if RLIMIT_DATA is set; also this member is tested to
> find a symbolic name of mmap event for perf system (we choose
> if event is generated for "heap" area); one more aplication is
> selinux -- we test if a process has PROCESS__EXECHEAP permission
> if trying to make heap area being executable with mprotect() syscall;
>
> - @brk is a current value for brk() syscall which lays inside heap
> area, it's shown in /proc/$pid/stat. When syscall brk() succesfully
> provides new memory area to a user space upon brk() completion the
> mm::brk is updated to carry new value;
>
> Both @start_brk and @brk are actively used in /proc/$pid/maps
> and /proc/$pid/smaps output to find a symbolic name "heap" for
> VMA being scanned;
>
> - @start_stack is printed out in /proc/$pid/stat and used to
> find a symbolic name "stack" for task and threads in
> /proc/$pid/maps and /proc/$pid/smaps output, and as the same
> as with @start_brk -- perf system uses it for event naming.
> Also kernel treat this member as a start address of where
> to map vDSO pages and to check if there is enough space
> for shmat() syscall;
>
> - @arg_start, @arg_end, @env_start and @env_end are printed out
> in /proc/$pid/stat. Another access to the data these members
> represent is to read /proc/$pid/environ or /proc/$pid/cmdline.
> Any attempt to read these areas kernel tests with access_process_vm
> helper so a user must have enough rights for this action;
>
> - @auxv and @auxv_size may be read from /proc/$pid/auxv. Strictly
> speaking kernel doesn't care much about which exactly data is
> sitting there because it is solely for userspace;
>
> - @exe_fd is referred from /proc/$pid/exe and when generating
> coredump. We uses prctl_set_mm_exe_file_locked helper to update
> this member, so exe-file link modification remains one-shot
> action.
>
> Still note that updating exe-file link now doesn't require sys-resource
> capability anymore, after all there is no much profit in preventing setup
> own file link (there are a number of ways to execute own code -- ptrace,
> ld-preload, so that the only reliable way to find which exactly code
> is executed is to inspect running program memory). Still we require
> the caller to be at least user-namespace root user.
>
> I believe the old interface should be deprecated and ripped off
> in a couple of kernel releases if no one against.
>
> To test if new interface is implemented in the kernel one
> can pass PR_SET_MM_MAP_SIZE opcode and the kernel returns
> the size of currently supported struct prctl_mm_map.
>
> v2:
> - compact macros (by keescook@)
> - wrap new code with CONFIG_ (by akpm@)
>
> v3 (by jln@):
> - use __prctl_check_order for brk and start_brk
> - use may_adjust_brk helper
> - make sure that only root can update @exe_fd link
>
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andrew Vagin <avagin@openvz.org>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Serge Hallyn <serge.hallyn@canonical.com>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> Cc: Vasiliy Kulikov <segoon@openwall.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Julien Tinnes <jln@google.com>
> ---
> include/uapi/linux/prctl.h | 25 +++++
> kernel/sys.c | 211 ++++++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 235 insertions(+), 1 deletion(-)
>
> Index: linux-2.6.git/include/uapi/linux/prctl.h
> ===================================================================
> --- linux-2.6.git.orig/include/uapi/linux/prctl.h
> +++ linux-2.6.git/include/uapi/linux/prctl.h
> @@ -119,6 +119,31 @@
> # define PR_SET_MM_ENV_END 11
> # define PR_SET_MM_AUXV 12
> # define PR_SET_MM_EXE_FILE 13
> +# define PR_SET_MM_MAP 14
> +# define PR_SET_MM_MAP_SIZE 15
> +
> +/*
> + * This structure provides new memory descriptor
> + * map which mostly modifies /proc/pid/stat[m]
> + * output for a task. This mostly done in a
> + * sake of checkpoint/restore functionality.
> + */
> +struct prctl_mm_map {
> + __u64 start_code; /* code section bounds */
> + __u64 end_code;
> + __u64 start_data; /* data section bounds */
> + __u64 end_data;
> + __u64 start_brk; /* heap for brk() syscall */
> + __u64 brk;
> + __u64 start_stack; /* stack starts at */
> + __u64 arg_start; /* command line arguments bounds */
> + __u64 arg_end;
> + __u64 env_start; /* environment variables bounds */
> + __u64 env_end;
> + __u64 *auxv; /* auxiliary vector */
> + __u32 auxv_size; /* vector size */
> + __u32 exe_fd; /* /proc/$pid/exe link file */
> +};
>
> /*
> * Set specific pid that is allowed to ptrace the current task.
> Index: linux-2.6.git/kernel/sys.c
> ===================================================================
> --- linux-2.6.git.orig/kernel/sys.c
> +++ linux-2.6.git/kernel/sys.c
> @@ -1687,6 +1687,208 @@ exit:
> return err;
> }
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +/*
> + * WARNING: we don't require any capability here so be very careful
> + * in what is allowed for modification from userspace.
> + */
> +static int validate_prctl_map_locked(struct prctl_mm_map *prctl_map)
> +{
> + unsigned long mmap_max_addr = TASK_SIZE;
> + struct mm_struct *mm = current->mm;
> + struct vm_area_struct *stack_vma;
> + int error = 0;
> +
> + /*
> + * Make sure the members are not somewhere outside
> + * of allowed address space.
> + */
> +#define __prctl_check_addr_space(__member) \
> + ({ \
> + int __rc; \
> + if ((unsigned long)prctl_map->__member < mmap_max_addr && \
> + (unsigned long)prctl_map->__member >= mmap_min_addr) \
> + __rc = 0; \
> + else \
> + __rc = -EINVAL; \
> + __rc; \
> + })
> + error |= __prctl_check_addr_space(start_code);
> + error |= __prctl_check_addr_space(end_code);
> + error |= __prctl_check_addr_space(start_data);
> + error |= __prctl_check_addr_space(end_data);
> + error |= __prctl_check_addr_space(start_stack);
> + error |= __prctl_check_addr_space(start_brk);
> + error |= __prctl_check_addr_space(brk);
> + error |= __prctl_check_addr_space(arg_start);
> + error |= __prctl_check_addr_space(arg_end);
> + error |= __prctl_check_addr_space(env_start);
> + error |= __prctl_check_addr_space(env_end);
> + if (error)
> + goto out;
> +#undef __prctl_check_addr_space
> +
> + /*
> + * Stack, brk, command line arguments and environment must exist.
> + */
> + stack_vma = find_vma(mm, (unsigned long)prctl_map->start_stack);
> + if (!stack_vma) {
> + error = -EINVAL;
> + goto out;
> + }
> +#define __prctl_check_vma(__member) \
> + find_vma(mm, (unsigned long)prctl_map->__member) ? 0 : -EINVAL
> + error |= __prctl_check_vma(start_brk);
> + error |= __prctl_check_vma(brk);
> + error |= __prctl_check_vma(arg_start);
> + error |= __prctl_check_vma(arg_end);
> + error |= __prctl_check_vma(env_start);
> + error |= __prctl_check_vma(env_end);
> + if (error)
> + goto out;
> +#undef __prctl_check_vma
> +
> + /*
> + * Make sure the pairs are ordered.
> + */
> +#define __prctl_check_order(__m1, __op, __m2) \
> + ((unsigned long)prctl_map->__m1 __op \
> + (unsigned long)prctl_map->__m2) ? 0 : -EINVAL
> + error |= __prctl_check_order(start_code, <, end_code);
> + error |= __prctl_check_order(start_data, <, end_data);
> + error |= __prctl_check_order(start_brk, <=, brk);
> + error |= __prctl_check_order(arg_start, <, arg_end);
> + error |= __prctl_check_order(env_start, <, env_end);
> + if (error)
> + goto out;
> +#undef __prctl_check_order
> +
> + error = -EINVAL;
> +
> + /*
> + * @brk should be after @end_data in traditional maps.
> + */
> + if (prctl_map->start_brk <= prctl_map->end_data ||
> + prctl_map->brk <= prctl_map->end_data)
> + goto out;
> +
> + /*
> + * Neither we should allow to override limits if they set.
> + */
> + if (may_adjust_brk(rlimit(RLIMIT_DATA), prctl_map->brk,
> + prctl_map->start_brk, prctl_map->end_data,
> + prctl_map->start_data))
> + goto out;
> +
> +#ifdef CONFIG_STACK_GROWSUP
> + if (may_adjust_brk(rlimit(RLIMIT_STACK),
> + stack_vma->vm_end,
> + prctl_map->start_stack, 0, 0))
> +#else
> + if (may_adjust_brk(rlimit(RLIMIT_STACK),
> + prctl_map->start_stack,
> + stack_vma->vm_start, 0, 0))
> +#endif
> + goto out;

Ah! Sorry, I missed this use of may_adjust_brk here. Perhaps rename
it, since we're not checking brk here, and pass the RLIMIT_* value to
the function, which can look it up itself? "check_vma_rlimit" ?

> +
> + /*
> + * Someone is trying to cheat the auxv vector.
> + */
> + if (prctl_map->auxv_size) {
> + if (!prctl_map->auxv ||
> + prctl_map->auxv_size > sizeof(mm->saved_auxv))
> + goto out;
> + }
> +
> + /*
> + * Finally, make sure the caller has the rights to
> + * change /proc/pid/exe link: only local root should
> + * be allowed to.
> + */
> + if (prctl_map->exe_fd != (u32)-1) {
> + struct user_namespace *ns = current_user_ns();
> + const struct cred *cred = current_cred();
> +
> + if (!uid_eq(cred->uid, make_kuid(ns, 0)) ||
> + !gid_eq(cred->gid, make_kgid(ns, 0)))
> + goto out;
> + }

I got tricked for a moment here. :) I see that even if we pass this
check, prctl_set_mm_exe_file will still do the additional checks too
during prctl_set_mm_map. Excellent!

> +
> + error = 0;
> +out:
> + return error;
> +}
> +
> +static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data_size)
> +{
> + struct prctl_mm_map prctl_map = { .exe_fd = (u32)-1, };
> + unsigned long user_auxv[AT_VECTOR_SIZE];
> + struct mm_struct *mm = current->mm;
> + int error = -EINVAL;
> +
> + BUILD_BUG_ON(sizeof(user_auxv) != sizeof(mm->saved_auxv));
> +
> + if (opt == PR_SET_MM_MAP_SIZE)
> + return put_user((unsigned int)sizeof(prctl_map),
> + (unsigned int __user *)addr);
> +
> + if (data_size != sizeof(prctl_map))
> + return -EINVAL;
> +
> + if (copy_from_user(&prctl_map, addr, sizeof(prctl_map)))
> + return -EFAULT;
> +
> + down_read(&mm->mmap_sem);
> +
> + if (validate_prctl_map_locked(&prctl_map))
> + goto out;
> +
> + if (prctl_map.auxv_size) {
> + up_read(&mm->mmap_sem);
> + memset(user_auxv, 0, sizeof(user_auxv));
> + error = copy_from_user(user_auxv,
> + (const void __user *)prctl_map.auxv,
> + prctl_map.auxv_size);
> + down_read(&mm->mmap_sem);
> + if (error)
> + goto out;
> + }
> +
> + if (prctl_map.exe_fd != (u32)-1) {
> + error = prctl_set_mm_exe_file_locked(mm, prctl_map.exe_fd);
> + if (error)
> + goto out;
> + }
> +
> + if (prctl_map.auxv_size) {
> + /* Last entry always must be AT_NULL */
> + user_auxv[AT_VECTOR_SIZE - 2] = AT_NULL;
> + user_auxv[AT_VECTOR_SIZE - 1] = AT_NULL;
> +
> + task_lock(current);
> + memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
> + task_unlock(current);
> + }
> +
> + mm->start_code = prctl_map.start_code;
> + mm->end_code = prctl_map.end_code;
> + mm->start_data = prctl_map.start_data;
> + mm->end_data = prctl_map.end_data;
> + mm->start_brk = prctl_map.start_brk;
> + mm->brk = prctl_map.brk;
> + mm->start_stack = prctl_map.start_stack;
> + mm->arg_start = prctl_map.arg_start;
> + mm->arg_end = prctl_map.arg_end;
> + mm->env_start = prctl_map.env_start;
> + mm->env_end = prctl_map.env_end;
> +
> + error = 0;
> +out:
> + up_read(&mm->mmap_sem);
> + return error;
> +}
> +#endif /* CONFIG_CHECKPOINT_RESTORE */
> +
> static int prctl_set_mm(int opt, unsigned long addr,
> unsigned long arg4, unsigned long arg5)
> {
> @@ -1695,9 +1897,16 @@ static int prctl_set_mm(int opt, unsigne
> struct vm_area_struct *vma;
> int error;
>
> - if (arg5 || (arg4 && opt != PR_SET_MM_AUXV))
> + if (arg5 || (arg4 && (opt != PR_SET_MM_AUXV &&
> + opt != PR_SET_MM_MAP &&
> + opt != PR_SET_MM_MAP_SIZE)))
> return -EINVAL;
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> + if (opt == PR_SET_MM_MAP || opt == PR_SET_MM_MAP_SIZE)
> + return prctl_set_mm_map(opt, (const void __user *)addr, arg4);
> +#endif
> +
> if (!capable(CAP_SYS_RESOURCE))
> return -EPERM;
>
>

I think this is looking good. Thanks for the refactoring!

-Kees

--
Kees Cook
Chrome OS Security


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