CVE-2021-46988: Linux Kernel

Medium severity, CVSS 5.5. EPSS: 0.2% chance of exploitation in the next 30 days.

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.

Affected products

• Linux Linux Kernel: from 4.11, before 4.14.233 (fixed in 4.14.233); from 4.15, before 4.19.191 (fixed in 4.19.191); from 4.20, before 5.4.120 (fixed in 5.4.120); from 5.5, before 5.10.38 (fixed in 5.10.38); from 5.11, before 5.11.22 (fixed in 5.11.22); from 5.12, before 5.12.5 (fixed in 5.12.5); …

Published 2024-02-28. Last modified 2026-06-17.