CVE-2026-63795

In the Linux kernel, the following vulnerability has been resolved:

9p: avoid putting oldfid in p9_client_walk() error path

When p9_client_walk() is called with clone set to false, fid aliases oldfid. If the walk subsequently fails after the request has been sent, the error path jumps to clunk_fid, which currently calls p9_fid_put(fid) unconditionally.

This drops a reference to oldfid even though ownership of oldfid remains with the caller. If this is the last reference, oldfid can be clunked and destroyed while the caller still expects it to be valid. A later use or put of oldfid can then trigger a use-after-free or refcount underflow.

Fix this by only putting fid in the clunk_fid error path when it does not alias oldfid, matching the existing guard in the error path below.

This can be triggered when a multi-component walk is split into multiple p9_client_walk() calls and a later non-cloning walk fails. A reproducer and refcount warning logs are available on request.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/1a3860d46e3eb47dbd60339783cdad7904486b9f
• https://git.kernel.org/stable/c/6dbe9443d9f5f7fb6d319a7b77108853ae6c6bea
• https://git.kernel.org/stable/c/99c379ca1e221c3d75c7c804ebbf4e5ee37a3070
• https://git.kernel.org/stable/c/a61bdcba4f64c2f90d01461913f429ab151f1ca6
• https://git.kernel.org/stable/c/a7656d368265d085ac9bb85ab31b0cdb72ad8c38
• https://git.kernel.org/stable/c/b84f46179c806450b89821221ea5bd9a1698aba8