Re: FreeBSD's RST validation

[Don.Lewis () TSC TDK COM (Don Lewis)]

On Aug 31,  1:26pm, Bruce A. Mah wrote:
} Subject: Re: FreeBSD's RST validation
} If memory serves me right, Don Lewis wrote:
}
} > On Aug 31, 11:24am, Bruce A. Mah wrote:
}
} [snip]
}
} > }                   if (tiflags & TH_RST) {
} > } !                         if ((tiflags & TH_ACK) &&
} > } !                         /* XXX outside window? XXX */
} > } !                             (SEQ_GT(ti->ti_ack, tp->iss) &&
} > } !                              SEQ_LEQ(ti->ti_ack, tp->snd_max)))
} > }                                   tp = tcp_drop(tp, ECONNREFUSED);
} > }                           goto drop;
} > }                   }
} >
} > As more data is sent across the connection, the wider the window for
} > a spoofed RST opens.  Once you send 2 GB, legitimate RSTs no longer
} > work.  You should probably be comparing against tp->snd_una instead
} > of tp->iss.
}
} Hmmm.  I was thinking specifically of the problem that with a RST arriving for
} a connection in SYN_SENT, the ACK in the RST-bearing segment has to
} acknowledge the initial SYN (thus, a test against tp->iss).  I hadn't thought
} that the ever-increasing difference between tp->snd_una and tp->iss would be a
} problem, since at this point in the code, we know that the receiving end of
} the connection is in SYN_SENT, as opposed to, say, ESTABLISHED.  Shouldn't
} (tp->snd_una == tp->iss) in this state, in which case, either would do?  (Not
} trying to split hairs, but just trying to learn a little more.)

Hmn, it's been a while since I looked at this stuff.  Yup, I didn't notice
that this was the SYN_SENT state and was thinking this was ESTABLISHED.

Now that I look at this change some more, I think your added tests are
a NOP because of the code just above this:

                if ((tiflags & TH_ACK) &&
                    (SEQ_LEQ(ti->ti_ack, tp->iss) ||
                     SEQ_GT(ti->ti_ack, tp->snd_max))) {
                        [ snip comment ]
                        if (taop->tao_ccsent != 0)
                                goto drop;
                        else
                                goto dropwithreset;

If the ACK is outside the window, the packet will already have been
dropped before we even look for the RST flag.