{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2021-46921", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2024-02-25T13:45:52.719Z", "datePublished": "2024-02-27T09:36:26.461Z", "dateUpdated": "2026-08-05T08:45:05.258Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-08-05T08:45:05.258Z" }, "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()]" } ], "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." } ] } ], "affected": [ { "product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "kernel/locking/qrwlock.c" ], "versions": [ { "version": "b519b56e378ee82caf9b079b04f5db87dedc3251", "lessThan": "5902f9453a313be8fe78cbd7e7ca9dba9319fc6e", "status": "affected", "versionType": "git" }, { "version": "b519b56e378ee82caf9b079b04f5db87dedc3251", "lessThan": "82808cc026811fbc3ecf0c0b267a12a339eead56", "status": "affected", "versionType": "git" }, { "version": "b519b56e378ee82caf9b079b04f5db87dedc3251", "lessThan": "82fa9ced35d88581cffa4a1c856fc41fca96d80a", "status": "affected", "versionType": "git" }, { "version": "b519b56e378ee82caf9b079b04f5db87dedc3251", "lessThan": "d558fcdb17139728347bccc60a16af3e639649d2", "status": "affected", "versionType": "git" }, { "version": "b519b56e378ee82caf9b079b04f5db87dedc3251", "lessThan": "84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896", "status": "affected", "versionType": "git" } ] }, { "product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "kernel/locking/qrwlock.c" ], "versions": [ { "version": "4.15", "status": "affected" }, { "version": "0", "lessThan": "4.15", "status": "unaffected", "versionType": "semver" }, { "version": "4.19.189", "lessThanOrEqual": "4.19.*", "status": "unaffected", "versionType": "semver" }, { "version": "5.4.115", "lessThanOrEqual": "5.4.*", "status": "unaffected", "versionType": "semver" }, { "version": "5.10.33", "lessThanOrEqual": "5.10.*", "status": "unaffected", "versionType": "semver" }, { "version": "5.11.17", "lessThanOrEqual": "5.11.*", "status": "unaffected", "versionType": "semver" }, { "version": "5.12", "lessThanOrEqual": "*", "status": "unaffected", "versionType": "original_commit_for_fix" } ] } ], "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" } ] } ] } ], "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" } ], "title": "locking/qrwlock: Fix ordering in queued_write_lock_slowpath()", "x_generator": { "engine": "bippy-1.2.0" } }, "adp": [ { "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" } } } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2024-07-05T17:22:01.427Z" } }, { "providerMetadata": { "orgId": "af854a3a-2127-422b-91ae-364da2661108", "shortName": "CVE", "dateUpdated": "2024-08-04T05:17:42.848Z" }, "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" ] } ] } ] } }