Subject: Re: [PATCH] Fix corrupt TCP packets when options space overflows with MD5SIG enabled

From: Stephen Hemminger <[email-masked]>
Date: Fri, 30 May 2008 13:32:36 -0700

On Fri, 30 May 2008 12:17:12 -0700
"Adam Langley" <[email-masked]> wrote:

> When MD5 signatures are turned on we can end up with syntactically invalid
> packets with a header length < 20 bytes. This is because tcp_header_size
> overflows with 12 bytes of timestamp, 20 bytes of signature and > 8 bytes of
> SACK option.
> 
> Since we can't fit any SACK blocks in the final 8 bytes of options space, and
> the MD5 signature is more important, we disable including SACK, or even
> advertising it, when MD5 is enabled.
> 
> Signed-off-by: Adam Langley <[email-masked]>
> 
> ---
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index e399bde..c98a4c9 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -477,6 +477,8 @@ static int tcp_transmit_skb(struct sock *sk,
> struct sk_buff *skb, int clone_it,

Your mailer is wrapping lines so patch is munged.

>  #ifdef CONFIG_TCP_MD5SIG
>         struct tcp_md5sig_key *md5;
>         __u8 *md5_hash_location;
> +#else
> +       void *const md5 = NULL;
>  #endif

You can just do:

+	struct tcp_md5sig_key *md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
-	struct tcp_md5sig_key *md5;
 	__u8 *md5_hash_location;
 #endif

GCC does the right thing.

>         struct tcphdr *th;
>         int sysctl_flags;
> @@ -504,6 +506,16 @@ static int tcp_transmit_skb(struct sock *sk,
> struct sk_buff *skb, int clone_it,
>         tcb = TCP_SKB_CB(skb);
>         tcp_header_size = tp->tcp_header_len;
> 
> +#ifdef CONFIG_TCP_MD5SIG
> +       /*
> +        * Are we doing MD5 on this segment? If so - make
> +        * room for it.
> +        */
> +       md5 = tp->af_specific->md5_lookup(sk, sk);
> +       if (md5)
> +               tcp_header_size += TCPOLEN_MD5SIG_ALIGNED;
> +#endif
> +
>  #define SYSCTL_FLAG_TSTAMPS    0x1
>  #define SYSCTL_FLAG_WSCALE     0x2
>  #define SYSCTL_FLAG_SACK       0x4
> @@ -519,12 +531,15 @@ static int tcp_transmit_skb(struct sock *sk,
> struct sk_buff *skb, int clone_it,
>                         tcp_header_size += TCPOLEN_WSCALE_ALIGNED;
>                         sysctl_flags |= SYSCTL_FLAG_WSCALE;
>                 }
> -               if (sysctl_tcp_sack) {
> +               /* We don't include SACK options if we are going to
> +                * include an MD5 signature because they can't fit
> +                * in the options space */
> +               if (sysctl_tcp_sack && !md5) {
>                         sysctl_flags |= SYSCTL_FLAG_SACK;
>                         if (!(sysctl_flags & SYSCTL_FLAG_TSTAMPS))
>                                 tcp_header_size += TCPOLEN_SACKPERM_ALIGNED;
>                 }
> -       } else if (unlikely(tp->rx_opt.eff_sacks)) {
> +       } else if (unlikely(tp->rx_opt.eff_sacks && !md5)) {
>                 /* A SACK is 2 pad bytes, a 2 byte header, plus
>                  * 2 32-bit sequence numbers for each SACK block.
>                  */
> @@ -536,16 +551,6 @@ static int tcp_transmit_skb(struct sock *sk,
> struct sk_buff *skb, int clone_it,
>         if (tcp_packets_in_flight(tp) == 0)
>                 tcp_ca_event(sk, CA_EVENT_TX_START);
> 
> -#ifdef CONFIG_TCP_MD5SIG
> -       /*
> -        * Are we doing MD5 on this segment? If so - make
> -        * room for it.
> -        */
> -       md5 = tp->af_specific->md5_lookup(sk, sk);
> -       if (md5)
> -               tcp_header_size += TCPOLEN_MD5SIG_ALIGNED;
> -#endif
> -
>         skb_push(skb, tcp_header_size);
>         skb_reset_transport_header(skb);
>         skb_set_owner_w(skb, sk);
> --
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