Re: [PATCH] mm, mmap: fix vma_merge() case 7 with vma_ops->close

[Lorenzo Stoakes]

On Thu, Feb 22, 2024 at 05:55:50PM +0100, Vlastimil Babka wrote:
> When debugging issues with a workload using SysV shmem, Michal Hocko has
> come up with a reproducer that shows how a series of mprotect()
> operations can result in an elevated shm_nattch and thus leak of the
> resource.
>
> The problem is caused by wrong assumptions in vma_merge() commit
> 714965ca8252 ("mm/mmap: start distinguishing if vma can be removed in
> mergeability test"). The shmem vmas have a vma_ops->close callback
> that decrements shm_nattch, and we remove the vma without calling it.
>
> vma_merge() has thus historically avoided merging vma's with
> vma_ops->close and commit 714965ca8252 was supposed to keep it that way.
> It relaxed the checks for vma_ops->close in can_vma_merge_after()
> assuming that it is never called on a vma that would be a candidate for
> removal. However, the vma_merge() code does also use the result of this
> check in the decision to remove a different vma in the merge case 7.
>
> A robust solution would be to refactor vma_merge() code in a way that
> the vma_ops->close check is only done for vma's that are actually going
> to be removed, and not as part of the preliminary checks. That would
> both solve the existing bug, and also allow additional merges that the
> checks currently prevent unnecessarily in some cases.

Let's do that pretty soon :) this is a bit of an ugly fix but
understandable to do it in this form to make it easier to backport (+
perhaps generate some CVEs? :)

>
> However to fix the existing bug first with a minimized risk, and for
> easier stable backports, this patch only adds a vma_ops->close check to
> the buggy case 7 specifically. All other cases of vma removal are
> covered by the can_vma_merge_before() check that includes the test for
> vma_ops->close.

I concur, all the other cases require merge_next which would have invoked
can_vma_merge_before() that calls is_mergeable_vma() with may_remove_vma
set to true hence performs the close check.

>
> The reproducer code, adapted from Michal Hocko's code:
>
> int main(int argc, char *argv[]) {
>   int segment_id;
>   size_t segment_size = 20 * PAGE_SIZE;
>   char * sh_mem;
>   struct shmid_ds shmid_ds;
>
>   key_t key = 0x1234;
>   segment_id = shmget(key, segment_size,
>                       IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
>   sh_mem = (char *)shmat(segment_id, NULL, 0);
>
>   mprotect(sh_mem + 2*PAGE_SIZE, PAGE_SIZE, PROT_NONE);
>
>   mprotect(sh_mem + PAGE_SIZE, PAGE_SIZE, PROT_WRITE);
>
>   mprotect(sh_mem + 2*PAGE_SIZE, PAGE_SIZE, PROT_WRITE);
>
>   shmdt(sh_mem);
>
>   shmctl(segment_id, IPC_STAT, &shmid_ds);
>   printf("nattch after shmdt(): %lu (expected: 0)\n", shmid_ds.shm_nattch);
>
>   if (shmctl(segment_id, IPC_RMID, 0))
>           printf("IPCRM failed %d\n", errno);
>   return (shmid_ds.shm_nattch) ? 1 : 0;
> }
>
> Fixes: 714965ca8252 ("mm/mmap: start distinguishing if vma can be removed in mergeability test")
> Reported-by: Michal Hocko <mhocko@xxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/mmap.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d89770eaab6b..a4238373ee9b 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -954,10 +954,19 @@ static struct vm_area_struct
>  	} else if (merge_prev) {			/* case 2 */
>  		if (curr) {
>  			vma_start_write(curr);
> -			err = dup_anon_vma(prev, curr, &anon_dup);
>  			if (end == curr->vm_end) {	/* case 7 */
> +				/*
> +				 * can_vma_merge_after() assumed we would not be
> +				 * removing prev vma, so it skipped the check
> +				 * for vm_ops->close, but we are removing curr
> +				 */
> +				if (curr->vm_ops && curr->vm_ops->close)
> +					err = -EINVAL;
> +				else
> +					err = dup_anon_vma(prev, curr, &anon_dup);
>  				remove = curr;
>  			} else {			/* case 5 */
> +				err = dup_anon_vma(prev, curr, &anon_dup);

This (ironically) duplicates code, could we pull this out of the if/else
and put it afterwards like:

	if (!err)
		err = dup_anon_vma(prev, curr, &anon_dup);

>  				adjust = curr;
>  				adj_start = (end - curr->vm_start);
>  			}
> --
> 2.43.1
>