Messages in this thread | | | Date | Sun, 02 Apr 2017 15:25:06 +0300 | From | Denys Fedoryshchenko <> | Subject | Re: KASAN, xt_TCPMSS finally found nasty use-after-free bug? 4.10.8 |
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On 2017-04-02 15:19, Eric Dumazet wrote: > On Sun, 2017-04-02 at 04:54 -0700, Eric Dumazet wrote: >> On Sun, 2017-04-02 at 13:45 +0200, Florian Westphal wrote: >> > Eric Dumazet <eric.dumazet@gmail.com> wrote: >> > > - for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) { >> > > + for (i = sizeof(struct tcphdr); i < tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) { >> > > if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) { >> > > u_int16_t oldmss; >> > >> > maybe I am low on caffeeine but this looks fine, for tcp header with >> > only tcpmss this boils down to "20 <= 24 - 4" so we acccess offsets 20-23 which seems ok. >> >> I am definitely low on caffeine ;) >> >> An issue in this function is that we might add the missing MSS option, >> without checking that TCP options are already full. >> >> But this should not cause a KASAN splat, only some malformed TCP >> packet >> >> (tcph->doff would wrap) > > Something like that maybe. > > diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c > index > 27241a767f17b4b27d24095a31e5e9a2d3e29ce4..1465aaf0e3a15d69d105d0a50b0429b11b6439d3 > 100644 > --- a/net/netfilter/xt_TCPMSS.c > +++ b/net/netfilter/xt_TCPMSS.c > @@ -151,7 +151,9 @@ tcpmss_mangle_packet(struct sk_buff *skb, > */ > if (len > tcp_hdrlen) > return 0; > - > + /* tcph->doff is 4 bits wide, do not wrap its value to 0 */ > + if (tcp_hdrlen >= 15 * 4) > + return 0; > /* > * MSS Option not found ?! add it.. > */ I will add also WARN_ON_ONCE(tcp_hdrlen >= 15 * 4) before, for curiosity, if this condition are triggered. Is it fine like that?
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