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.
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.
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") Signed-off-by: Vlastimil Babka vbabka@suse.cz Reported-by: Michal Hocko mhocko@suse.com Cc: Liam R. Howlett Liam.Howlett@oracle.com Cc: Lorenzo Stoakes lstoakes@gmail.com Cc: stable@vger.kernel.org --- v2: deduplicate code, per Lorenzo mm/mmap.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/mm/mmap.c b/mm/mmap.c index d89770eaab6b..3281287771c9 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -954,13 +954,21 @@ 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; remove = curr; } else { /* case 5 */ adjust = curr; adj_start = (end - curr->vm_start); } + if (!err) + err = dup_anon_vma(prev, curr, &anon_dup); } } else { /* merge_next */ vma_start_write(next);