CVE-2021-46921

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()]

Gravité et risque d'exploitation

CVE-2021-46921 obtient un score CVSS de 7.8/10, soit une gravité élevée. Selon le modèle EPSS, la probabilité qu'elle soit exploitée dans les 30 prochains jours est de 0.24 %, ce qui la place devant 13.8 % des CVE référencées.

Que faire ?

• Vérifiez si Linux (versions b519b56e378ee82caf9b079b04f5db87dedc3251, <d558fcdb17139728347bccc60a16af3e639649d2, <=5.11.*, 0, <=5.10.*, 5.12, <84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896, 5.4.115, <5902f9453a313be8fe78cbd7e7ca9dba9319fc6e, <=5.4.*, <82fa9ced35d88581cffa4a1c856fc41fca96d80a, 4.19.189, <=4.19.*, 5.10.33, <=*, <4.15, 5.11.17, 4.15, <82808cc026811fbc3ecf0c0b267a12a339eead56) est présent sur votre parc.
• Appliquez le correctif ou la version corrigée indiqués dans les références de l'éditeur.
• À défaut de correctif, limitez l'exposition (filtrage réseau, désactivation du composant).
• Automatisez la détection : les agents CVE Heimdall (gratuits, open source) corrèlent l'inventaire de vos serveurs avec la base CVE.

Références

• 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