Re: [PATCH] ipv4: tcp_input: fix stack out of bounds when parsing TCP options.

[Eric Dumazet]

On Wed, May 29, 2019 at 1:10 AM Young Xiao <92siuyang@xxxxxxxxx> wrote:
>
> The TCP option parsing routines in tcp_parse_options function could
> read one byte out of the buffer of the TCP options.
>
> 1         while (length > 0) {
> 2                 int opcode = *ptr++;
> 3                 int opsize;
> 4
> 5                 switch (opcode) {
> 6                 case TCPOPT_EOL:
> 7                         return;
> 8                 case TCPOPT_NOP:        /* Ref: RFC 793 section 3.1 */
> 9                         length--;
> 10                        continue;
> 11                default:
> 12                        opsize = *ptr++; //out of bound access
>
> If length = 1, then there is an access in line2.
> And another access is occurred in line 12.
> This would lead to out-of-bound access.
>
> Therefore, in the patch we check that the available data length is
> larger enough to pase both TCP option code and size.
>
> Signed-off-by: Young Xiao <92siuyang@xxxxxxxxx>
> ---
>  net/ipv4/tcp_input.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 20f6fac..9775825 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3791,6 +3791,8 @@ void tcp_parse_options(const struct net *net,
>                         length--;
>                         continue;
>                 default:
> +                       if (length < 2)
> +                               return;
>                         opsize = *ptr++;
>                         if (opsize < 2) /* "silly options" */
>                                 return;

In practice we are good, since we have at least 320 bytes of room there,
and the test done later catches silly options.

if (opsize < 2) /* "silly options" */
    return;
if (opsize > length)   /* remember, opsize >= 2 here */
     return; /* don't parse partial options */

I guess adding yet another conditional will make this code obviously
correct for all eyes
and various tools.

Thanks.

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>