[PATCH net-next 1/3] ipv6: sr: Fix MAC comparison to be constant-time

[PATCH net-next 1/3] ipv6: sr: Fix MAC comparison to be constant-time

[Eric Biggers]

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>
---
 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);
-- 
2.50.1

Re: [PATCH net-next 1/3] ipv6: sr: Fix MAC comparison to be constant-time

[Andrea Mayer]

On Fri, 15 Aug 2025 20:11:34 -0700
Eric Biggers <ebiggers@kernel.org> 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>
> ---
>  net/ipv6/seg6_hmac.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 

Hi Eric,

Thanks for the fix!

I believe it would be best to submit this fix separately from the current patch
set. Since this addresses a bug rather than an enhancement or cleanup, sending
it individually with the 'net' tag will help facilitate applying this patch to
the net tree.

Ciao,
Andrea

> 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);
> -- 
> 2.50.1
> 

Re: [PATCH net-next 1/3] ipv6: sr: Fix MAC comparison to be constant-time

[Eric Biggers]

On Mon, Aug 18, 2025 at 09:16:07PM +0200, Andrea Mayer wrote:
> On Fri, 15 Aug 2025 20:11:34 -0700
> Eric Biggers <ebiggers@kernel.org> 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>
> > ---
> >  net/ipv6/seg6_hmac.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> 
> Hi Eric,
> 
> Thanks for the fix!
> 
> I believe it would be best to submit this fix separately from the current patch
> set. Since this addresses a bug rather than an enhancement or cleanup, sending
> it individually with the 'net' tag will help facilitate applying this patch to
> the net tree.
> 
> Ciao,
> Andrea

Then there would be a merge conflict between the two patchsets.

I can do that if you want, but then I'd probably have to wait for this
patch to reach net-next before the rest can proceed.

- Eric

Re: [PATCH net-next 1/3] ipv6: sr: Fix MAC comparison to be constant-time

[Eric Biggers]

On Mon, Aug 18, 2025 at 07:38:52PM +0000, Eric Biggers wrote:
> On Mon, Aug 18, 2025 at 09:16:07PM +0200, Andrea Mayer wrote:
> > On Fri, 15 Aug 2025 20:11:34 -0700
> > Eric Biggers <ebiggers@kernel.org> 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>
> > > ---
> > >  net/ipv6/seg6_hmac.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > 
> > Hi Eric,
> > 
> > Thanks for the fix!
> > 
> > I believe it would be best to submit this fix separately from the current patch
> > set. Since this addresses a bug rather than an enhancement or cleanup, sending
> > it individually with the 'net' tag will help facilitate applying this patch to
> > the net tree.
> > 
> > Ciao,
> > Andrea
> 
> Then there would be a merge conflict between the two patchsets.
> 
> I can do that if you want, but then I'd probably have to wait for this
> patch to reach net-next before the rest can proceed.

I guess patches 2 and 3 have to wait for "ipv6: sr: validate HMAC
algorithm ID in seg6_hmac_info_add" anyway, as they conflict with that
too.  Okay, I resent patch 1 as a standalone patch targeting "net":
https://lore.kernel.org/netdev/20250818202724.15713-1-ebiggers@kernel.org/

- Eric