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

[Eric Dumazet]

On Wed, May 29, 2019 at 8:11 AM Yang Xiao <92siuyang@xxxxxxxxx> wrote:
>
> Indeed, condition opsize < 2 and opsize > length can deduce that length >= 2.
> However, before the condition (if opsize < 2), there may be one-byte
> out-of-bound access in line 12.
> I'm not sure whether I have put it very clearly.

Maybe I should have been clear about the 320 bytes we have at the end
of skb->head

This is the struct skb_shared_info

So reading one byte, 'out-of-bound' here is harmless.

Whatever value is read, we will return early without ever looking at a
following byte.


>
> On Wed, May 29, 2019 at 10:20 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > 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>
>
>
>
> --
> Best regards!
>
> Young
> -----------------------------------------------------------