[PATCH] userfaultfd: release page in error path to avoid BUG_ON

[Axel Rasmussen]

Consider the following sequence of events (described from the point of
view of the commit that introduced the bug - see "Fixes:" below):

1. Userspace issues a UFFD ioctl, which ends up calling into
   shmem_mcopy_atomic_pte(). We successfully account the blocks, we
   shmem_alloc_page(), but then the copy_from_user() fails. We return
   -EFAULT. We don't release the page we allocated.
2. Our caller detects this error code, tries the copy_from_user() after
   dropping the mmap_sem, and retries, calling back into
   shmem_mcopy_atomic_pte().
3. Meanwhile, let's say another process filled up the tmpfs being used.
4. So shmem_mcopy_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 -EFAULT is returned.

(Later on in the commit history, -EFAULT became -ENOENT, mmap_sem became
mmap_lock, and shmem_inode_acct_block() was added.)

A malicious user (even an unprivileged one) could trigger this
intentionally without too much trouble.

To fix this, detect if we have a "dangling" page when accounting fails,
and if so, release it before returning.

Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY")
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
 mm/shmem.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 26c76b13ad23..46766c9d7151 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2375,8 +2375,19 @@ 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)) {
+			unlock_page(*pagep);
+			put_page(*pagep);
+			*pagep = NULL;
+		}
 		goto out;
+	}
 
 	if (!*pagep) {
 		page = shmem_alloc_page(gfp, info, pgoff);
-- 
2.31.1.498.g6c1eba8ee3d-goog