The freeing and re-allocation of algorithm are protected by cpool_mutex,
so it doesn't fix an actual use-after-free, but avoids a deserved
refcount_warn_saturate() warning.

A trivial fix for the racy behavior.

Fixes: 8c73b26315aa ("net/tcp: Prepare tcp_md5sig_pool for TCP-AO")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dmitry Safonov <redacted>
---
 net/ipv4/tcp_sigpool.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_sigpool.c b/net/ipv4/tcp_sigpool.c
index 55b310a722c7..8512cb09ebc0 100644
--- a/net/ipv4/tcp_sigpool.c
+++ b/net/ipv4/tcp_sigpool.c

@@ -162,9 +162,8 @@ int tcp_sigpool_alloc_ahash(const char *alg, size_t scratch_size)
 		if (strcmp(cpool[i].alg, alg))
 			continue;
 
-		if (kref_read(&cpool[i].kref) > 0)
-			kref_get(&cpool[i].kref);
-		else
+		/* pairs with tcp_sigpool_release() */
+		if (!kref_get_unless_zero(&cpool[i].kref))
 			kref_init(&cpool[i].kref);
 		ret = i;
 		goto out;

---
base-commit: 1a44b0073b9235521280e19d963b6dfef7888f18
change-id: 20231222-tcp-ao-kref_get_unless_zero-fe7105781ba4