[PATCH net v2] ipv6: sr: Fix MAC comparison to be constant-time

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

Fixes: bf355b8d2c30 ("ipv6: sr: add core files for SR HMAC support")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---

v2: sent as standalone patch targeting net instead of net-next.

 net/ipv6/seg6_hmac.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/seg6_hmac.c b/net/ipv6/seg6_hmac.c
index f78ecb6ad8383..5dae892bbc73b 100644
--- a/net/ipv6/seg6_hmac.c
+++ b/net/ipv6/seg6_hmac.c

@@ -33,10 +33,11 @@
 #include <net/ip6_route.h>
 #include <net/addrconf.h>
 #include <net/xfrm.h>
 
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 #include <net/seg6.h>
 #include <net/genetlink.h>
 #include <net/seg6_hmac.h>
 #include <linux/random.h>
 

@@ -278,11 +279,11 @@ bool seg6_hmac_validate_skb(struct sk_buff *skb)
 		return false;
 
 	if (seg6_hmac_compute(hinfo, srh, &ipv6_hdr(skb)->saddr, hmac_output))
 		return false;
 
-	if (memcmp(hmac_output, tlv->hmac, SEG6_HMAC_FIELD_LEN) != 0)
+	if (crypto_memneq(hmac_output, tlv->hmac, SEG6_HMAC_FIELD_LEN))
 		return false;
 
 	return true;
 }
 EXPORT_SYMBOL(seg6_hmac_validate_skb);

base-commit: 715c7a36d59f54162a26fac1d1ed8dc087a24cf1
-- 
2.50.1

On Mon, 18 Aug 2025 13:27:24 -0700
Eric Biggers [off-list ref] wrote:

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

Fixes: bf355b8d2c30 ("ipv6: sr: add core files for SR HMAC support")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---

v2: sent as standalone patch targeting net instead of net-next.

 net/ipv6/seg6_hmac.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

The fix looks good to me. Thanks!

Ciao,
Andrea

Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski [off-list ref]:

On Mon, 18 Aug 2025 13:27:24 -0700 you wrote:

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

Fixes: bf355b8d2c30 ("ipv6: sr: add core files for SR HMAC support")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

[...]

Here is the summary with links:
  - [net,v2] ipv6: sr: Fix MAC comparison to be constant-time
    https://git.kernel.org/netdev/net/c/a458b2902115

You are awesome, thank you!
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