# CVE-2021-47396: linux: In the Linux kernel, the following vulnerability has been resolved:  mac80211-hwsim: fix late beacon hrtimer handling...

CVE-2021-47396 is fixed in linux on 1 platform, first by DEBIAN-CVE-2021-47396, released May 21, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2021-47396
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0024
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 21, 2024

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.14.12-1 | [DEBIAN-CVE-2021-47396](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47396.md) | May 21, 2024 |
| Linux | linux Debian 13.5 (trixie) | 5.14.12-1 | [DEBIAN-CVE-2021-47396](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47396.md) | May 21, 2024 |
| Linux | linux Debian 14 | 5.14.12-1 | [DEBIAN-CVE-2021-47396](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47396.md) | May 21, 2024 |

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