CVE-2021-47584

/// Technical 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

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References