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SubjectRe: [RFC PATCH] binfmt_elf: Protect mm_struct access with mmap_sem
On Wed, Jun 12, 2019 at 04:28:11PM +0200, Michal Koutný wrote:
> find_extend_vma assumes the caller holds mmap_sem as a reader (explained
> in expand_downwards()). The path when we are extending the stack VMA to
> accomodate argv[] pointers happens without the lock.
>
> I was not able to cause an mm_struct corruption but
> BUG_ON(!rwsem_is_locked(&mm->mmap_sem)) in find_extend_vma could be
> triggered as
>
> # <bigfile xargs echo
> xargs: echo: terminated by signal 11
>
> (bigfile needs to have more than RLIMIT_STACK / sizeof(char *) rows)
>
> Other accesses to mm_struct in exec path are protected by mmap_sem, so
> conservatively, protect also this one. Besides that, explain why we omit
> mm_struct.arg_lock in the exec(2) path.
>
> Cc: Cyrill Gorcunov <gorcunov@gmail.com>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> ---
>
> When I was attempting to reduce usage of mmap_sem I came across this
> unprotected access and increased number of its holders :-/
>
> I'm not sure whether there is a real concurrent writer at this early
> stages (I considered khugepaged especially as setup_arg_pages invokes
> khugepaged_enter_vma_merge but we're lucky because khugepaged skips it
> because of VM_STACK_INCOMPLETE_SETUP).
>
> A nicer approach would perhaps be to do all this exec setup when the
> mm_struct is still not exposed via current->mm (and hence no need to
> synchronize via mmap_sem). But I didn't look enough into binfmt specific
> whether it is even doable and worth it.
>
> So I'm sending this for a discussion.
>
> fs/binfmt_elf.c | 10 +++++++++-
> fs/exec.c | 3 ++-
> 2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 8264b468f283..48e169760a9c 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -299,7 +299,11 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
> * Grow the stack manually; some architectures have a limit on how
> * far ahead a user-space access may be in order to grow the stack.
> */
> + if (down_read_killable(&current->mm->mmap_sem))
> + return -EINTR;
> vma = find_extend_vma(current->mm, bprm->p);
> + up_read(&current->mm->mmap_sem);
> +

Good catch, Michal! Actually the loader code is heavy on its own so
I think having readlock taken here should not cause any perf problems
but worth having for consistency.

Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>

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