Commit 1a3860d46e3e for kernel

commit 1a3860d46e3eb47dbd60339783cdad7904486b9f
Author: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Date:   Thu May 28 13:39:16 2026 +0800

    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.

    Fixes: b48dbb998d70 ("9p fid refcount: add p9_fid_get/put wrappers")
    Cc: stable@vger.kernel.org
    Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
    Reported-by: Ao Wang <wangao@seu.edu.cn>
    Reported-by: Xuewei Feng <fengxw06@126.com>
    Reported-by: Qi Li <qli01@tsinghua.edu.cn>
    Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
    Assisted-by: GLM 5.1
    Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
    Message-ID: <20260528053918.53550-1-zhaoyz24@mails.tsinghua.edu.cn>
    Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

diff --git a/net/9p/client.c b/net/9p/client.c
index 69d3efd340c0..ef64546c6d52 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1094,7 +1094,8 @@ struct p9_fid *p9_client_walk(struct p9_fid *oldfid, uint16_t nwname,

 clunk_fid:
 	kfree(wqids);
-	p9_fid_put(fid);
+	if (fid != oldfid)
+		p9_fid_put(fid);
 	fid = NULL;

 error: