CVE-2026-63795

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

9p: avoid putting oldfid in p9clientwalk() error path

When p9clientwalk() 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 clunkfid, which currently calls p9fid_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 p9clientwalk() calls and a later non-cloning walk fails. A reproducer and refcount warning logs are available on request.