In the Linux kernel, the following vulnerability has been resolved:userfaultfd: release page in error path to avoid BUG_ONConsider the following sequence of events: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.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().Meanwhile, let's say another process filled up the tmpfs being used.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.