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
>
