CVE-2021-46988: userfaultfd: release page in error path to avoid BUG_ON

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-02-28 · updated 2026-06-17
Weakness: CWE-416

Description

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 kernel: from 4.11 before 4.14.233; from 4.15 before 4.19.191; from 4.20 before 5.4.120; from 5.5 before 5.10.38; from 5.11 before 5.11.22; from 5.12 before 5.12.5 — fixed in 4.14.233, 4.19.191, 5.10.38, 5.11.22, 5.12.5, 5.4.120

How to fix

Upgrade to a fixed version: Linux kernel 4.14.233, 4.19.191, 5.10.38, 5.11.22. Check the vendor advisory for mitigations.

References

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