CVE-2021-46988

In the Linux kernel, the following vulnerability has been resolved:
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.