[PATCH] sunrpc: use kref_get_unless_zero in auth_domain_lookup

[Jeff Layton <jlayton@kernel.org>]

auth_domain_put() uses kref_put_lock(), which atomically decrements the
refcount before acquiring auth_domain_lock. This creates a window where
an auth_domain entry is still linked on the hash list with refcount == 0.

auth_domain_lookup() walks the hash under auth_domain_lock but uses plain
kref_get() to acquire a reference. If it finds an entry in this transient
zero-refcount state, refcount_inc() triggers a WARN and refuses to
increment (saturating refcount_t semantics), but the function returns the
pointer anyway. The caller then holds a dangling reference: when the
concurrent auth_domain_put() finally acquires the lock and runs
auth_domain_release(), the object is freed while the lookup caller still
has a pointer to it.

The sibling function auth_domain_find() already handles this correctly
using kref_get_unless_zero(). Apply the same pattern in
auth_domain_lookup(): treat a zero-refcount entry as absent and continue
searching. The loop then either finds another live entry or falls through
to insert the new domain, preserving existing semantics.

Reported-by: Chris Mason <clm@meta.com>
Assisted-by: kres:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 net/sunrpc/svcauth.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/sunrpc/svcauth.c b/net/sunrpc/svcauth.c
index 55b4d2874188..8e01f0626759 100644
--- a/net/sunrpc/svcauth.c
+++ b/net/sunrpc/svcauth.c
@@ -245,8 +245,10 @@ auth_domain_lookup(char *name, struct auth_domain *new)
 	spin_lock(&auth_domain_lock);
 
 	hlist_for_each_entry(hp, head, hash) {
-		if (strcmp(hp->name, name)==0) {
-			kref_get(&hp->ref);
+		if (strcmp(hp->name, name) == 0) {
+			if (!kref_get_unless_zero(&hp->ref))
+				continue;
+
 			spin_unlock(&auth_domain_lock);
 			return hp;
 		}

---
base-commit: 508c9eaa7e0b952c4fe019880796e6207e3cd201
change-id: 20260520-nfsd-fixes-f137572d0480

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>