locking/qrwlock: Fix ordering in queued_write_lock_slowpath()

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Summary

CVECVE-2021-46921
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2024-02-27 10:15:06 UTC
Updated2026-08-04 10:16:36 UTC
DescriptionIn 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()]

Risk And Classification

Primary CVSS: v3.1 5.5 MEDIUM from [email protected]

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Problem Types: CWE-668

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary5.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
3.1416baaa9-dc9f-4396-8d5f-8c081fb06d67Secondary7.8HIGHCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
3.1CNADECLARED7.8HIGHCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Operating System Linux Linux Kernel All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected b519b56e378ee82caf9b079b04f5db87dedc3251 5902f9453a313be8fe78cbd7e7ca9dba9319fc6e git Not specified
CNA Linux Linux affected b519b56e378ee82caf9b079b04f5db87dedc3251 82808cc026811fbc3ecf0c0b267a12a339eead56 git Not specified
CNA Linux Linux affected b519b56e378ee82caf9b079b04f5db87dedc3251 82fa9ced35d88581cffa4a1c856fc41fca96d80a git Not specified
CNA Linux Linux affected b519b56e378ee82caf9b079b04f5db87dedc3251 d558fcdb17139728347bccc60a16af3e639649d2 git Not specified
CNA Linux Linux affected b519b56e378ee82caf9b079b04f5db87dedc3251 84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896 git Not specified
CNA Linux Linux affected 4.15 Not specified
CNA Linux Linux unaffected 4.15 semver Not specified
CNA Linux Linux unaffected 4.19.189 4.19.* semver Not specified
CNA Linux Linux unaffected 5.4.115 5.4.* semver Not specified
CNA Linux Linux unaffected 5.10.33 5.10.* semver Not specified
CNA Linux Linux unaffected 5.11.17 5.11.* semver Not specified
CNA Linux Linux unaffected 5.12 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896 af854a3a-2127-422b-91ae-364da2661108 git.kernel.org Patch
git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e af854a3a-2127-422b-91ae-364da2661108 git.kernel.org Patch
git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2 af854a3a-2127-422b-91ae-364da2661108 git.kernel.org Patch
git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56 af854a3a-2127-422b-91ae-364da2661108 git.kernel.org Patch
git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a af854a3a-2127-422b-91ae-364da2661108 git.kernel.org Patch
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Legacy QID Mappings

  • 755988 SUSE Enterprise Linux Security Update for the Linux Kernel (SUSE-SU-2024:0975-1)
  • 756005 SUSE Enterprise Linux Security Update for the Linux Kernel (SUSE-SU-2024:0925-1)

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