Re: [RFC PATCH] binfmt_elf: Protect mm_struct access with mmap_sem

[Cyrill Gorcunov <gorcunov@gmail.com>]

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>