CVE-2021-46921

Advisory lineage Upstream: 0 Downstream: 8
Modified
Published: 27 Feb 2024, 09:36
Last modified:11 May 2026, 13:44

Vulnerability Summary

Overall Risk (default)
low
22/100
CVSS Score
5.5 MEDIUM
v3.1 (nvd)
EPSS Score
0.02% LOW
0% probability 0.00%
KEV
Not listed
Ransomware
No reports
Public exploits
None found
Dark Web
Not detected

Timeline

27 Feb 2024, 09:36
Published
Vulnerability first disclosed
11 May 2026, 13:44
Last Modified
Vulnerability information updated

Description

In the Linux kernel, the following vulnerability has been resolved: locking/qrwlock: Fix ordering in queued_write_lock_slowpath() While this code is executed with the wait_lock held, a reader can acquire the lock without holding wait_lock. The writer side loops checking the value with the atomic_cond_read_acquire(), but only truly acquires the lock when the compare-and-exchange is completed successfully which isn’t ordered. This exposes the window between the acquire and the cmpxchg to an A-B-A problem which allows reads following the lock acquisition to observe values speculatively before the write lock is truly acquired. We've seen a problem in epoll where the reader does a xchg while holding the read lock, but the writer can see a value change out from under it. Writer | Reader -------------------------------------------------------------------------------- ep_scan_ready_list() | |- write_lock_irq() | |- queued_write_lock_slowpath() | |- atomic_cond_read_acquire() | | read_lock_irqsave(&ep->lock, flags); --> (observes value before unlock) | chain_epi_lockless() | | epi->next = xchg(&ep->ovflist, epi); | | read_unlock_irqrestore(&ep->lock, flags); | | | atomic_cmpxchg_relaxed() | |-- READ_ONCE(ep->ovflist); | A core can order the read of the ovflist ahead of the atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire semantics addresses this issue at which point the atomic_cond_read can be switched to use relaxed semantics. [peterz: use try_cmpxchg()]

CVSS Metrics

  • v3.1•MEDIUM•Score: 5.5CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

EPSS Trends

Current EPSS score: 0.02%• Percentile: 4%

Techniques & Countermeasures

  • CWE-668•Exposure of Resource to Wrong Sphere

    The product exposes a resource to the wrong control sphere, providing unintended actors with inappropriate access to the resource.

Affected Systems

  • linux•linux

    ≥ b519b56e378ee82caf9b079b04f5db87dedc3251, < 5902f9453a313be8fe78cbd7e7ca9dba9319fc6e | ≥ b519b56e378ee82caf9b079b04f5db87dedc3251, < 82808cc026811fbc3ecf0c0b267a12a339eead56 | ≥ b519b56e378ee82caf9b079b04f5db87dedc3251, < 82fa9ced35d88581cffa4a1c856fc41fca96d80a | ≥ b519b56e378ee82caf9b079b04f5db87dedc3251, < d558fcdb17139728347bccc60a16af3e639649d2 | ≥ b519b56e378ee82caf9b079b04f5db87dedc3251, < 84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896 | 4.15

  • linux•linux_kernel

    ≥ 4.15.0, < 4.19.189 | ≥ 4.20.0, < 5.4.115 | ≥ 5.5.0, < 5.10.33 | ≥ 5.11.0, < 5.11.17

References (5)