FAILED: patch "[PATCH] nfsd: release path refs on follow_down() error" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 6cba08dc1922140d260cfeb30bbda4ee1bf869d8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090324-reprogram-throwaway-926d@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 6cba08dc1922140d260cfeb30bbda4ee1bf869d8 Mon Sep 17 00:00:00 2001
From: Chris Mason <clm@meta.com>
Date: Sun, 31 May 2026 08:06:59 -0400
Subject: [PATCH] nfsd: release path refs on follow_down() error

nfsd_cross_mnt() initializes a local struct path with mntget() and
dget() before calling follow_down(). On a negative return the error
arm jumps to out without releasing those references:

    err = follow_down(&path, follow_flags);
    if (err < 0)
            goto out;

follow_down() never drops the caller's entry-time refs on any error
sub-case; for example a pre-cross d_manage() failure leaves path
untouched, so the mntget()/dget() taken on entry survive the call.

Every other early-exit arm in nfsd_cross_mnt() (other-namespace
return, IS_ERR(exp2), and the success tail after the swap) already
calls path_put(&path); the err < 0 arm is the lone omission. The
leak inflates mnt_count and d_count on each failed cross-mount,
blocking umount and pinning dentries against the shrinker, and is
reachable by any authenticated NFS client through nfsd_lookup_dentry
or the NFSv4 READDIR encode path.

Fix by calling path_put(&path) before the goto out in the err < 0
arm so the entry-time refs are released on all follow_down() error
returns.

Fixes: cc53ce53c869 ("Add a dentry op to allow processes to be held during pathwalk transit")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260531-nfsd-testing-v1-2-7bfa481b0540@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0dcfaab95021..f73012bc742f 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -137,8 +137,10 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 		follow_flags = LOOKUP_AUTOMOUNT;
 
 	err = follow_down(&path, follow_flags);
-	if (err < 0)
+	if (err < 0) {
+		path_put(&path);
 		goto out;
+	}
 	if (path.mnt == exp->ex_path.mnt && path.dentry == dentry &&
 	    nfsd_mountpoint(dentry, exp) == 2) {
 		/* This is only a mountpoint in some other namespace */