# CVE-2021-47584: linux: In the Linux kernel, the following vulnerability has been resolved:  iocost: Fix divide-by-zero on donation from low...

CVE-2021-47584 is fixed in linux on 1 platform, first by DEBIAN-CVE-2021-47584, released June 19, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2021-47584
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0025
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 19, 2024

## Description

In the Linux kernel, the following vulnerability has been resolved:  iocost: Fix divide-by-zero on donation from low hweight cgroup  The donation calculation logic assumes that the donor has non-zero after-donation hweight, so the lowest active hweight a donating cgroup can have is 2 so that it can donate 1 while keeping the other 1 for itself. Earlier, we only donated from cgroups with sizable surpluses so this condition was always true. However, with the precise donation algorithm implemented, f1de2439ec43 ("blk-iocost: revamp donation amount determination") made the donation amount calculation exact enabling even low hweight cgroups to donate.  This means that in rare occasions, a cgroup with active hweight of 1 can enter donation calculation triggering the following warning and then a divide-by-zero oops.   WARNING: CPU: 4 PID: 0 at block/blk-iocost.c:1928 transfer_surpluses.cold+0x0/0x53 [884/94867]  ...  RIP: 0010:transfer_surpluses.cold+0x0/0x53  Code: 92 ff 48 c7 c7 28 d1 ab b5 65 48 8b 34 25 00 ae 01 00 48 81 c6 90 06 00 00 e8 8b 3f fe ff 48 c7 c0 ea ff ff ff e9 95 ff 92 ff <0f> 0b 48 c7 c7 30 da ab b5 e8 71 3f fe ff 4c 89 e8 4d 85 ed 74 0 4  ...  Call Trace:   <IRQ>   ioc_timer_fn+0x1043/0x1390   call_timer_fn+0xa1/0x2c0   __run_timers.part.0+0x1ec/0x2e0   run_timer_softirq+0x35/0x70  ...  iocg: invalid donation weights in /a/b: active=1 donating=1 after=0  Fix it by excluding cgroups w/ active hweight < 2 from donating. Excluding these extreme low hweight donations shouldn't affect work conservation in any meaningful way.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.15.15-1 | [DEBIAN-CVE-2021-47584](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47584.md) | Jun 19, 2024 |
| Linux | linux Debian 13.5 (trixie) | 5.15.15-1 | [DEBIAN-CVE-2021-47584](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47584.md) | Jun 19, 2024 |
| Linux | linux Debian 14 | 5.15.15-1 | [DEBIAN-CVE-2021-47584](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47584.md) | Jun 19, 2024 |

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