FreeBSD Security Advisory FreeBSD-SA-05:01.telnet

[Colin Percival]

Steve Kiernan wrote:
> I was looking at this patch, but there seems to be an error in it:
> 
>  unsigned char slc_reply[128];
> +unsigned char const * const slc_reply_eom = &slc_reply[sizeof(slc_reply)];
>  unsigned char *slc_replyp;
> 
> Should the value for slc_reply_eom not be this instead?
> 
> unsigned char const * const slc_reply_eom = &slc_reply[sizeof(slc_reply) - 1];

No.

> Considering the conditionals are the following:
> 
> +       if (&slc_replyp[6+2] > slc_reply_eom)
> +               return;
> 
> .. and ..
> 
> +    /* The end of negotiation command requires 2 bytes. */
> +    if (&slc_replyp[2] > slc_reply_eom)
> +            return;
> 
> If you don't subtract 1 from the sizeof(slc_reply) or change the
> conditional operators to >=, then you could try to write one byte past
> the end of the buffer.

The tests are written a bit oddly, but I'm fairly certain that they
are correct.  &slc_replyp[6+2] and &slc_replyp[2] are not the
addresses of the last bytes which will be written; rather, they are
the addresses of the byte after the last byte which will be written.

Taking the second example, if slc_replyp == slc_reply + 126, then we
will have &slc_replyp[2] == slc_reply_eom, but (looking at the code)
the two final bytes will be written into slc_reply[126] and
slc_reply[127].

Colin Percival