CVE-2021-47396: mac80211-hwsim: fix late beacon hrtimer handling

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-05-21 · updated 2026-06-17

Description

In the Linux kernel, the following vulnerability has been resolved: mac80211-hwsim: fix late beacon hrtimer handling Thomas explained in https://lore.kernel.org/r/87mtoeb4hb.ffs@tglx that our handling of the hrtimer here is wrong: If the timer fires late (e.g. due to vCPU scheduling, as reported by Dmitry/syzbot) then it tries to actually rearm the timer at the next deadline, which might be in the past already: 1 2 3 N N+1 | | | ... | | ^ intended to fire here (1) ^ next deadline here (2) ^ actually fired here The next time it fires, it's later, but will still try to schedule for the next deadline (now 3), etc. until it catches up with N, but that might take a long time, causing stalls etc. Now, all of this is simulation, so we just have to fix it, but note that the behaviour is wrong even per spec, since there's no value then in sending all those beacons unaligned - they should be aligned to the TBTT (1, 2, 3, ... in the picture), and if we're a bit (or a lot) late, then just resume at that point. Therefore, change the code to use hrtimer_forward_now() which will ensure that the next firing of the timer would be at N+1 (in the picture), i.e. the next interval point after the current time.

Affected products

Linux kernel: from 3.9 before 5.4.151; from 5.5 before 5.10.71; from 5.11 before 5.14.10; 5.15-rc1, 5.15-rc2, 5.15-rc3 — fixed in 5.10.71, 5.14.10, 5.4.151

How to fix

Upgrade to a fixed version: Linux kernel 5.10.71, 5.14.10, 5.4.151. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2021-47396 or the next vulnerability affects your stack