[PATCH 5.10 201/289] userfaultfd: release page in error path to avoid BUG_ON

From: Greg Kroah-Hartman
Date: Mon May 17 2021 - 11:59:24 EST

From: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>

commit 7ed9d238c7dbb1fdb63ad96a6184985151b0171c upstream.

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
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@xxxxxxxxxx
Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY")
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
mm/shmem.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2378,8 +2378,18 @@ static int shmem_mfill_atomic_pte(struct
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);