Subject: [PATCH 6.6] net/tcp-md5: Fix MAC comparison to be constant-time

From: Eric Biggers <[email-masked]>
Date: Tue, 10 Mar 2026 13:17:01 -0700

commit 46d0d6f50dab706637f4c18a470aac20a21900d3 upstream.

To prevent timing attacks, MACs need to be compared in constant
time.  Use the appropriate helper function for this.

Fixes: cfb6eeb4c860 ("[TCP]: MD5 Signature Option (RFC2385) support.")
Fixes: 658ddaaf6694 ("tcp: md5: RST: getting md5 key from listener")
Cc: [email-masked]
Signed-off-by: Eric Biggers <[email-masked]>
Link: https://patch.msgid.link/[email-masked]
Signed-off-by: Jakub Kicinski <[email-masked]>
---
 net/ipv4/tcp.c      | 3 ++-
 net/ipv4/tcp_ipv4.c | 3 ++-
 net/ipv6/tcp_ipv6.c | 3 ++-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 2bae34d63c3db..021e1bdbddcb8 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -242,10 +242,11 @@
  */
 
 #define pr_fmt(fmt) "TCP: " fmt
 
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/types.h>
 #include <linux/fcntl.h>
 #include <linux/poll.h>
@@ -4554,11 +4555,11 @@ tcp_inbound_md5_hash(const struct sock *sk, const struct sk_buff *skb,
 	else
 		genhash = tp->af_specific->calc_md5_hash(newhash,
 							 hash_expected,
 							 NULL, skb);
 
-	if (genhash || memcmp(hash_location, newhash, 16) != 0) {
+	if (genhash || crypto_memneq(hash_location, newhash, 16)) {
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMD5FAILURE);
 		if (family == AF_INET) {
 			net_info_ratelimited("MD5 Hash failed for (%pI4, %d)->(%pI4, %d)%s L3 index %d\n",
 					saddr, ntohs(th->source),
 					daddr, ntohs(th->dest),
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index ab4be34e58bb2..c8d35f1c0ece2 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -78,10 +78,11 @@
 #include <linux/seq_file.h>
 #include <linux/inetdevice.h>
 #include <linux/btf_ids.h>
 
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 #include <linux/scatterlist.h>
 
 #include <trace/events/tcp.h>
 
 #ifdef CONFIG_TCP_MD5SIG
@@ -774,11 +775,11 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb)
 		if (!key)
 			goto out;

 		genhash = tcp_v4_md5_hash_skb(newhash, key, NULL, skb);
-		if (genhash || memcmp(hash_location, newhash, 16) != 0)
+		if (genhash || crypto_memneq(hash_location, newhash, 16))
 			goto out;
 
 	}
 
 	if (key) {
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 624ab1424eba7..2c579868fe81f 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -62,10 +62,11 @@
 
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
 
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 #include <linux/scatterlist.h>
 
 #include <trace/events/tcp.h>
 
 static void	tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb);
@@ -1033,11 +1034,11 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 		key = tcp_v6_md5_do_lookup(sk1, &ipv6h->saddr, l3index);
 		if (!key)
 			goto out;
 
 		genhash = tcp_v6_md5_hash_skb(newhash, key, NULL, skb);
-		if (genhash || memcmp(hash_location, newhash, 16) != 0)
+		if (genhash || crypto_memneq(hash_location, newhash, 16))
 			goto out;
 	}
 #endif
 
 	if (th->ack)

base-commit: 4fc00fe35d46b4fc8dac2eb543a0e3d44bb15f47
-- 
2.53.0