[PATCH net] sctp: reject stale cookies with mismatched verification tags

[PATCH net] sctp: reject stale cookies with mismatched verification tags

[Yuxiang Yang]

sctp_unpack_cookie() skips cookie expiration checks whenever an
association already exists.  This is broader than the exception in
RFC 9260 Section 5.2.4.

For an existing association, Section 5.2.4 permits an expired State
Cookie only when both Verification Tags in the cookie match the current
association.  Otherwise, the packet SHOULD be discarded and a Stale
Cookie ERROR MUST be sent.

The broad check lets an expired Action A restart cookie reach
sctp_sf_do_dupcook_a().  In a runtime test with the default 60 second
cookie lifetime, replaying such a cookie after 65 seconds returned a
COOKIE-ACK and restarted the association.

Check cookie expiration unless both Verification Tags match.  This
preserves the Action D exception for a lost COOKIE ACK while rejecting
expired cookies in all other cases.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
---
 net/sctp/sm_make_chunk.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
index c02809264..a1c0334a1 100644
--- a/net/sctp/sm_make_chunk.c
+++ b/net/sctp/sm_make_chunk.c
@@ -1802,9 +1802,9 @@ struct sctp_association *sctp_unpack_cookie(
 		goto fail;
 	}
 
-	/* Check to see if the cookie is stale.  If there is already
-	 * an association, there is no need to check cookie's expiration
-	 * for init collision case of lost COOKIE ACK.
+	/* Check to see if the cookie is stale.  RFC 9260 Section 5.2.4
+	 * exempts an expired cookie only when both Verification Tags match
+	 * the current association.
 	 * If skb has been timestamped, then use the stamp, otherwise
 	 * use current time.  This introduces a small possibility that
 	 * a cookie may be considered expired, but this would only slow
@@ -1815,7 +1815,10 @@ struct sctp_association *sctp_unpack_cookie(
 	else
 		kt = ktime_get_real();
 
-	if (!asoc && ktime_before(bear_cookie->expiration, kt)) {
+	if ((!asoc ||
+	     asoc->c.my_vtag != bear_cookie->my_vtag ||
+	     asoc->c.peer_vtag != bear_cookie->peer_vtag) &&
+	    ktime_before(bear_cookie->expiration, kt)) {
 		suseconds_t usecs = ktime_to_us(ktime_sub(kt, bear_cookie->expiration));
 		__be32 n = htonl(usecs);
 
-- 
2.34.1

Re: [PATCH net] sctp: reject stale cookies with mismatched verification tags

[Xin Long]

On Thu, Jul 23, 2026 at 6:56 PM Yuxiang Yang
<yangyx22@mails.tsinghua.edu.cn> wrote:
>
> sctp_unpack_cookie() skips cookie expiration checks whenever an
> association already exists.  This is broader than the exception in
> RFC 9260 Section 5.2.4.
>
> For an existing association, Section 5.2.4 permits an expired State
> Cookie only when both Verification Tags in the cookie match the current
> association.  Otherwise, the packet SHOULD be discarded and a Stale
> Cookie ERROR MUST be sent.
>
> The broad check lets an expired Action A restart cookie reach
> sctp_sf_do_dupcook_a().  In a runtime test with the default 60 second
> cookie lifetime, replaying such a cookie after 65 seconds returned a
> COOKIE-ACK and restarted the association.
>
> Check cookie expiration unless both Verification Tags match.  This
> preserves the Action D exception for a lost COOKIE ACK while rejecting
> expired cookies in all other cases.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
> ---
>  net/sctp/sm_make_chunk.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> index c02809264..a1c0334a1 100644
> --- a/net/sctp/sm_make_chunk.c
> +++ b/net/sctp/sm_make_chunk.c
> @@ -1802,9 +1802,9 @@ struct sctp_association *sctp_unpack_cookie(
>                 goto fail;
>         }
>
> -       /* Check to see if the cookie is stale.  If there is already
> -        * an association, there is no need to check cookie's expiration
> -        * for init collision case of lost COOKIE ACK.
> +       /* Check to see if the cookie is stale.  RFC 9260 Section 5.2.4
> +        * exempts an expired cookie only when both Verification Tags match
> +        * the current association.
>          * If skb has been timestamped, then use the stamp, otherwise
>          * use current time.  This introduces a small possibility that
>          * a cookie may be considered expired, but this would only slow
> @@ -1815,7 +1815,10 @@ struct sctp_association *sctp_unpack_cookie(
>         else
>                 kt = ktime_get_real();
>
> -       if (!asoc && ktime_before(bear_cookie->expiration, kt)) {
> +       if ((!asoc ||
> +            asoc->c.my_vtag != bear_cookie->my_vtag ||
> +            asoc->c.peer_vtag != bear_cookie->peer_vtag) &&
> +           ktime_before(bear_cookie->expiration, kt)) {
>                 suseconds_t usecs = ktime_to_us(ktime_sub(kt, bear_cookie->expiration));
>                 __be32 n = htonl(usecs);
>
> --
> 2.34.1
>
Acked-by: Xin Long <lucien.xin@gmail.com>

Re: [PATCH net] sctp: reject stale cookies with mismatched verification tags

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 23 Jul 2026 22:56:23 +0000 you wrote:
> sctp_unpack_cookie() skips cookie expiration checks whenever an
> association already exists.  This is broader than the exception in
> RFC 9260 Section 5.2.4.
> 
> For an existing association, Section 5.2.4 permits an expired State
> Cookie only when both Verification Tags in the cookie match the current
> association.  Otherwise, the packet SHOULD be discarded and a Stale
> Cookie ERROR MUST be sent.
> 
> [...]

Here is the summary with links:
  - [net] sctp: reject stale cookies with mismatched verification tags
    https://git.kernel.org/netdev/net/c/9d8da8e0a9bc

You are awesome, thank you!
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