CVE-2021-46988: userfaultfd: release page in error path to avoid BUG_ON
Vulnerability summary
CVE-2021-46988 is a medium (CVSS 5.5) Use After Free vulnerability in linux. EPSS estimates a 0.24% chance of exploitation in the next 30 days, higher than 15.1% of scored vulnerabilities.
Not severe (CVSS 5.5, local access); no exploitation reported by the 5 checked sources
Source freshness unavailable
How is this decided?Advisory lineage
Upstream 0 · Downstream 6 · Related 6Timeline
- 28-02-2024PublishedVulnerability first disclosed.
- 11-05-2026Last modifiedVulnerability information updated.
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.
CVSS metrics
4 scores · 3 sources · 2 versions- OSV · UbuntuEcosystem advisory · calculated5.5MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- OSV · DebianEcosystem advisory · calculated5.5MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.24%
Chance of exploitation in the next 30 days
Percentile
15%
Versus all other CVEs
30-day change
No change 0.00 pp
From 0.24% on 04-10-2026
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Known conditions: local access, low-privilege account, no user interaction (CVSS vector). Other conditions: not stated in public data.
Ubuntu85 products
- linux< 4.15.0-151.157 | < 5.4.0-80.90
- linux-allwinner-5.19All versions (No fix recorded)
- linux-aws< 4.15.0-1109.116 | < 5.4.0-1054.57
- linux-aws-5.0All versions (No fix recorded)
- linux-aws-5.11All versions (No fix recorded)
Linux2 products
- linux≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < 319116227e52d49eee671f0aa278bac89b3c1b69 | ≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < 07c9b834c97d0fa3402fb7f3f3b32df370a6ff1f | ≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < b3f1731c6d7fbc1ebe3ed8eff6d6bec56d76ff43 | ≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < 140cfd9980124aecb6c03ef2e69c72d0548744de | ≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < ad53127973034c63b5348715a1043d0e80ceb330 | ≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < 2d59a0ed8b26b8f3638d8afc31f839e27759f1f6 | ≥ cb658a453b9327ce96ce5222c24d162b5b65b564, < 7ed9d238c7dbb1fdb63ad96a6184985151b0171c | 4.11
- linux_kernel≥ 4.11, < 4.14.233 | ≥ 4.15, < 4.19.191 | ≥ 4.20, < 5.4.120 | ≥ 5.5, < 5.10.38 | ≥ 5.11, < 5.11.22 | ≥ 5.12, < 5.12.5 | 5.13:rc1
Debian1 product
- linux< 5.10.38-1
3 vendors · 88 products
