Debian: CVE-2021-46921: linux -- security update

Debian: CVE-2021-46921: linux -- security update

Severity

5

CVSS

(AV:L/AC:L/Au:S/C:C/I:N/A:N)

Published

02/27/2024

Created

07/31/2024

Added

07/30/2024

Modified

01/30/2025

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()]

Solution(s)

• debian-upgrade-linux

References

• https://attackerkb.com/topics/cve-2021-46921
• CVE - 2021-46921