CVE-2021-47396 mac80211-hwsim: fix late beacon hrtimer handling
A flaw in the Linux kernel mac80211-hwsim module causes system stalls when beacon timers fire late, requiring a patch to prevent denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2021-47396?
CVE-2021-47396 is a medium-severity denial of service vulnerability in Linux kernel, published on 21 May 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2021-47396?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2021-47396?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2021-47396 being exploited?
There is no known exploitation of CVE-2021-47396 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as DEBIAN-CVE-2021-47396, GHSA-cwq4-fr9x-6844, UBUNTU-CVE-2021-47396.