Subject: Re: possible kernel oops from user MSS

From: Shan Wei <[email-masked]>
Date: Thu, 11 Nov 2010 13:15:01 +0800

David Miller wrote, at 11/11/2010 04:41 AM:
> From: Steve Chen <[email-masked]>
> Date: Wed, 10 Nov 2010 07:24:51 -0600
> 
>> With commit f5fff5dc8a7a3f395b0525c02ba92c95d42b7390, a user program
>> can pass in TCP_MAXSEG of 12 (or TCPOLEN_TSTAMP_ALIGNED), and cause
>> kernel oops with division by 0
>>  in tcp_select_initial_window.  One way to prevent it is to change the
>> minimum value for TCP_MAXSEG in do_tcp_setsockopt from 8 to some value
>> over 12.  Two questions.
>>
>> 1.  Is this the right solution?
>> 2.  If it is, what is a good minimum value?
> 
> Thanks Steve, I'll fix this like so:
> 
> --------------------
> tcp: Increase TCP_MAXSEG socket option minimum.
> 
> As noted by Steve Chen, since commit
> f5fff5dc8a7a3f395b0525c02ba92c95d42b7390 ("tcp: advertise MSS
> requested by user") we can end up with a situation where
> tcp_select_initial_window() does a divide by a zero (or
> even negative) mss value.
> 
> The problem is that sometimes we subtract TCPOLEN_TSTAMP_ALIGNED
> from the mss.
> 
> Fix this by increasing the minimum from 8 to 8 plus the value
> of TCPOLEN_TSTATMP_ALIGNED.

In tcp_connect_init(), if tcp_header_len includes TCPOLEN_TSTAMP_ALIGNED(12 bytes)
and TCPOLEN_MD5SIG_ALIGNED(20 bytes).

This fix is still not perfect.

The minimum value of TCP_MAXSEG is 20 tytes, tcp_select_initial_window() still be 
called with negative mss value.

-- 
Best Regards
-----
Shan Wei

> Reported-by: Steve Chen <[email-masked]>
> Signed-off-by: David S. Miller <[email-masked]>
> ---
>  net/ipv4/tcp.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 245603c..6b0eb4d 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2246,7 +2246,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>  		/* Values greater than interface MTU won't take effect. However
>  		 * at the point when this call is done we typically don't yet
>  		 * know which interface is going to be used */
> -		if (val < 8 || val > MAX_TCP_WINDOW) {
> +		if (val < TCPOLEN_TSTAMP_ALIGNED + 8 || val > MAX_TCP_WINDOW) {
>  			err = -EINVAL;
>  			break;
>  		}