{"id":"CVE-2021-46921","state":"PUBLISHED","title":"locking/qrwlock: Fix ordering in queued_write_lock_slowpath()","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nlocking/qrwlock: Fix ordering in queued_write_lock_slowpath()\n\nWhile this code is executed with the wait_lock held, a reader can\nacquire the lock without holding wait_lock.  The writer side loops\nchecking the value with the atomic_cond_read_acquire(), but only truly\nacquires the lock when the compare-and-exchange is completed\nsuccessfully which isn’t ordered. This exposes the window between the\nacquire and the cmpxchg to an A-B-A problem which allows reads\nfollowing the lock acquisition to observe values speculatively before\nthe write lock is truly acquired.\n\nWe've seen a problem in epoll where the reader does a xchg while\nholding the read lock, but the writer can see a value change out from\nunder it.\n\n  Writer                                | Reader\n  --------------------------------------------------------------------------------\n  ep_scan_ready_list()                  |\n  |- write_lock_irq()                   |\n      |- queued_write_lock_slowpath()   |\n\t|- atomic_cond_read_acquire()   |\n\t\t\t\t        | read_lock_irqsave(&ep->lock, flags);\n     --> (observes value before unlock) |  chain_epi_lockless()\n     |                                  |    epi->next = xchg(&ep->ovflist, epi);\n     |                                  | read_unlock_irqrestore(&ep->lock, flags);\n     |                                  |\n     |     atomic_cmpxchg_relaxed()     |\n     |-- READ_ONCE(ep->ovflist);        |\n\nA core can order the read of the ovflist ahead of the\natomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire\nsemantics addresses this issue at which point the atomic_cond_read can\nbe switched to use relaxed semantics.\n\n[peterz: use try_cmpxchg()]","publishedAt":"2024-02-27T09:36:26.461Z","updatedAt":"2026-08-05T08:45:05.258Z","severity":{"score":7.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"5902f9453a313be8fe78cbd7e7ca9dba9319fc6e","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"82808cc026811fbc3ecf0c0b267a12a339eead56","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"82fa9ced35d88581cffa4a1c856fc41fca96d80a","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"d558fcdb17139728347bccc60a16af3e639649d2","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/locking/qrwlock.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.15","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.15","versionType":"semver"},{"version":"4.19.189","status":"unaffected","lessThanOrEqual":"4.19.*","versionType":"semver"},{"version":"5.4.115","status":"unaffected","lessThanOrEqual":"5.4.*","versionType":"semver"},{"version":"5.10.33","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.11.17","status":"unaffected","lessThanOrEqual":"5.11.*","versionType":"semver"},{"version":"5.12","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/locking/qrwlock.c"]}],"references":["https://git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e","https://git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56","https://git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a","https://git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2","https://git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896"],"weaknesses":[],"ingestedAt":"2026-10-29T19:49:16.371Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2021/46xxx/CVE-2021-46921.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2021-46921","state":"PUBLISHED","datePublished":"2024-02-27T09:36:26.461Z","dateUpdated":"2026-08-05T08:45:05.258Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2024-02-25T13:45:52.719Z"},"containers":{"cna":{"title":"locking/qrwlock: Fix ordering in queued_write_lock_slowpath()","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nlocking/qrwlock: Fix ordering in queued_write_lock_slowpath()\n\nWhile this code is executed with the wait_lock held, a reader can\nacquire the lock without holding wait_lock.  The writer side loops\nchecking the value with the atomic_cond_read_acquire(), but only truly\nacquires the lock when the compare-and-exchange is completed\nsuccessfully which isn’t ordered. This exposes the window between the\nacquire and the cmpxchg to an A-B-A problem which allows reads\nfollowing the lock acquisition to observe values speculatively before\nthe write lock is truly acquired.\n\nWe've seen a problem in epoll where the reader does a xchg while\nholding the read lock, but the writer can see a value change out from\nunder it.\n\n  Writer                                | Reader\n  --------------------------------------------------------------------------------\n  ep_scan_ready_list()                  |\n  |- write_lock_irq()                   |\n      |- queued_write_lock_slowpath()   |\n\t|- atomic_cond_read_acquire()   |\n\t\t\t\t        | read_lock_irqsave(&ep->lock, flags);\n     --> (observes value before unlock) |  chain_epi_lockless()\n     |                                  |    epi->next = xchg(&ep->ovflist, epi);\n     |                                  | read_unlock_irqrestore(&ep->lock, flags);\n     |                                  |\n     |     atomic_cmpxchg_relaxed()     |\n     |-- READ_ONCE(ep->ovflist);        |\n\nA core can order the read of the ovflist ahead of the\natomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire\nsemantics addresses this issue at which point the atomic_cond_read can\nbe switched to use relaxed semantics.\n\n[peterz: use try_cmpxchg()]"}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"5902f9453a313be8fe78cbd7e7ca9dba9319fc6e","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"82808cc026811fbc3ecf0c0b267a12a339eead56","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"82fa9ced35d88581cffa4a1c856fc41fca96d80a","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"d558fcdb17139728347bccc60a16af3e639649d2","versionType":"git"},{"version":"b519b56e378ee82caf9b079b04f5db87dedc3251","status":"affected","lessThan":"84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/locking/qrwlock.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.15","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.15","versionType":"semver"},{"version":"4.19.189","status":"unaffected","lessThanOrEqual":"4.19.*","versionType":"semver"},{"version":"5.4.115","status":"unaffected","lessThanOrEqual":"5.4.*","versionType":"semver"},{"version":"5.10.33","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.11.17","status":"unaffected","lessThanOrEqual":"5.11.*","versionType":"semver"},{"version":"5.12","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/locking/qrwlock.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e"},{"url":"https://git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56"},{"url":"https://git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a"},{"url":"https://git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2"},{"url":"https://git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","baseScore":7.8,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:L - The bug is in queued_write_lock_slowpath() for rwlock_t; the demonstrated path is local epoll_wait → ep_send_events → ep_done_scan → write_lock_irq(), which requires local syscall access rather than network or physical reachability.\nAC:L - An unprivileged attacker can drive both sides of the race (epoll_wait as writer and concurrent fd events triggering ep_poll_callback as reader), forcing lock contention into the slowpath; Amazon observed this ordering failure in production on ARM, so success is not dependent on victim-controlled conditions.\nPR:L - epoll_create/epoll_ctl/epoll_wait and ordinary process activity that take rwlocks are available to unprivileged local users with no capabilities or user-namespace privilege escalation required.\nUI:N - Exploitation requires only attacker-controlled threads and file descriptors; no separate victim action such as mounting a filesystem or opening a crafted file is needed.\nS:U - Impact remains within the host kernel's privilege boundary (incorrect mutual exclusion and resulting memory/state corruption), with no VM-escape or other cross-authority scope change.\nC:H - Missing acquire semantics let a writer observe shared kernel state before the write lock is truly held, so protected structures (e.g. ep->ovflist and any other rwlock-guarded data) can be read incorrectly; as a generic rwlock break this enables broad kernel info disclosure via corrupted/racy structure access.\nI:H - The writer then mutates state based on that stale view (e.g. ep_done_scan writing EP_UNACTIVE_PTR over an overflow chain it missed), and broken write-lock exclusivity on widely used rwlocks such as tasklist_lock can corrupt kernel pointer-rich structures, enabling integrity loss and control-flow primitives.\nA:H - Data races from broken qrwlock acquire on structures protected by rwlock_t (epoll lists, tasklist, and others on ARM64/Android) can yield list corruption, use-after-free, and kernel oops/panic under attacker-triggered contention."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-08-05T08:45:05.258Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"4.19.189"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"5.4.115"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"5.10.33"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"5.11.17"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"5.12"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2021-46921","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"version":"2.0.3","timestamp":"2024-02-27T16:11:46.310286Z"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2024-07-05T17:22:01.427Z"}},{"title":"CVE Program Container","references":[{"url":"https://git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e","tags":["x_transferred"]},{"url":"https://git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56","tags":["x_transferred"]},{"url":"https://git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a","tags":["x_transferred"]},{"url":"https://git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2","tags":["x_transferred"]},{"url":"https://git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896","tags":["x_transferred"]}],"providerMetadata":{"shortName":"CVE","orgId":"af854a3a-2127-422b-91ae-364da2661108","dateUpdated":"2024-08-04T05:17:42.848Z"}}]}}}