diff options
authorAxel Rasmussen <axelrasmussen@google.com>2021-05-14 17:27:19 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2021-05-14 19:41:32 -0700
commit7ed9d238c7dbb1fdb63ad96a6184985151b0171c (patch)
parentd6e621de1fceb3b098ebf435ef7ea91ec4838a1a (diff)
userfaultfd: release page in error path to avoid BUG_ON
Consider the following sequence of events: 1. Userspace issues a UFFD ioctl, which ends up calling into shmem_mfill_atomic_pte(). We successfully account the blocks, we shmem_alloc_page(), but then the copy_from_user() fails. We return -ENOENT. We don't release the page we allocated. 2. Our caller detects this error code, tries the copy_from_user() after dropping the mmap_lock, and retries, calling back into shmem_mfill_atomic_pte(). 3. Meanwhile, let's say another process filled up the tmpfs being used. 4. So shmem_mfill_atomic_pte() fails to account blocks this time, and immediately returns - without releasing the page. This triggers a BUG_ON in our caller, which asserts that the page should always be consumed, unless -ENOENT is returned. To fix this, detect if we have such a "dangling" page when accounting fails, and if so, release it before returning. Link: https://lkml.kernel.org/r/20210428230858.348400-1-axelrasmussen@google.com Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY") Signed-off-by: Axel Rasmussen <axelrasmussen@google.com> Reported-by: Hugh Dickins <hughd@google.com> Acked-by: Hugh Dickins <hughd@google.com> Reviewed-by: Peter Xu <peterx@redhat.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 11 insertions, 1 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index eb131b9fb190..5d46611cba8d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2361,8 +2361,18 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
pgoff_t offset, max_off;
ret = -ENOMEM;
- if (!shmem_inode_acct_block(inode, 1))
+ if (!shmem_inode_acct_block(inode, 1)) {
+ /*
+ * We may have got a page, returned -ENOENT triggering a retry,
+ * and now we find ourselves with -ENOMEM. Release the page, to
+ * avoid a BUG_ON in our caller.
+ */
+ if (unlikely(*pagep)) {
+ put_page(*pagep);
+ *pagep = NULL;
+ }
goto out;
+ }
if (!*pagep) {
page = shmem_alloc_page(gfp, info, pgoff);

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