CVE-2021-46921: In the Linux kernel, the following vulnerability has been resolved: locking/qrwlock: Fix ordering in queued_write_lock_slowpath() While this code is executed…

PriorityP423medium5.5CVSS 3.1

AVLACLPRLUINSUCHINAN

EPSS

0.24%

14.6th percentile

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()]

Affected

15 ranges

Vendor	Product	Version range	Fixed in
debian	linux	< linux 5.10.38-1 (bookworm)	linux 5.10.38-1 (bookworm)
linux	linux	—	—
linux	linux	>= b519b56e378ee82caf9b079b04f5db87dedc3251 < 5902f9453a313be8fe78cbd7e7ca9dba9319fc6e	5902f9453a313be8fe78cbd7e7ca9dba9319fc6e
linux	linux	>= b519b56e378ee82caf9b079b04f5db87dedc3251 < 82808cc026811fbc3ecf0c0b267a12a339eead56	82808cc026811fbc3ecf0c0b267a12a339eead56
linux	linux	>= b519b56e378ee82caf9b079b04f5db87dedc3251 < 82fa9ced35d88581cffa4a1c856fc41fca96d80a	82fa9ced35d88581cffa4a1c856fc41fca96d80a
linux	linux	>= b519b56e378ee82caf9b079b04f5db87dedc3251 < d558fcdb17139728347bccc60a16af3e639649d2	d558fcdb17139728347bccc60a16af3e639649d2
linux	linux	>= b519b56e378ee82caf9b079b04f5db87dedc3251 < 84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896	84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 4.15.0 < 4.19.189	4.19.189
linux	linux_kernel	>= 4.20.0 < 5.4.115	5.4.115
linux	linux_kernel	>= 5.11.0 < 5.11.17	5.11.17
linux	linux_kernel	>= 5.5.0 < 5.10.33	5.10.33

CVSS provenance

nvdv3.15.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

osv5.5MEDIUM

vendor_debian5.5MEDIUM

vendor_redhat5.5MEDIUM

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