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
