On Sat, Dec 30, 2017 at 08:42:41AM +0100, Willem de Bruijn wrote: > > syzkaller hit the following crash on > > 37759fa6d0fa9e4d6036d19ac12f555bfc0aeafd > > git://git.cmpxchg.org/linux-mmots.git/master > > compiler: gcc (GCC) 7.1.1 20170620 > > .config is attached > > Raw console output is attached. > > C reproducer is attached > > syzkaller reproducer is attached. See https://goo.gl/kgGztJ > > for information about syzkaller reproducers > > Reproduced with the C reproducer on v4.15-rc1 and mainline > going back at least to v4.8, but not v4.7. SCTP GSO was > introduced in v4.8-rc1, so a patch in this set is likely the starting > point. Indeed crashes at 90017accff61 ("sctp: Add GSO support"), > but not at 90017accff61~4. > > The reproducer with its sandbox removed shows this invocation in strace -f > > # strace -f ./repro2 > [... skipped ...] > socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3 > open("/dev/net/tun", O_RDONLY) = 4 > fcntl(4, F_DUPFD, 3) = 5 > socket(PF_PACKET, SOCK_RAW|SOCK_CLOEXEC, 8) = 6 > ioctl(4, TUNSETIFF, 0x20e63000) = 0 > ioctl(3, SIOCSIFFLAGS, {ifr_name="syz0", > ifr_flags=IFF_UP|IFF_PROMISC|IFF_ALLMULTI}) = 0 > setsockopt(6, SOL_PACKET, 0xf /* PACKET_??? */, [4096], 4) = 0 > ioctl(6, SIOCGIFINDEX, {ifr_name="syz0", ifr_index=24}) = 0 > bind(6, {sa_family=AF_PACKET, proto=0000, if24, pkttype=PACKET_HOST, > addr(6)={1, aaaaaaaaaa00}, 20) = 0 > dup2(6, 5) = 5 > write(5, "\0\201\1\0\350\367\0\0\3\0E\364\0 \0d\0\0\7\2042\342\0\0\0 > \177\0\0\1\0\t"..., 42 > > where 0xf in setsockopt is PACKET_VNET_HDR > > So this is a packet socket writing something that apparently looks > like an SCTP packet, is only 42 bytes long, but has GSO set in its > virtio_net_hdr struct. > > It crashes in skb_segment seemingly on a NULL list_skb. > > (gdb) list *(skb_segment+0x2a4) > 0xffffffff8167cc24 is in skb_segment (net/core/skbuff.c:3566). > 3561 if (hsize < 0) > 3562 hsize = 0; > 3563 if (hsize > len || !sg) > 3564 hsize = len; > 3565 > 3566 if (!hsize && i >= nfrags && skb_headlen(list_skb) && > 3567 (skb_headlen(list_skb) == len || sg)) { > 3568 BUG_ON(skb_headlen(list_skb) > len); > 3569 > 3570 i = 0; > > Likely there is a hidden assumption about SCTP GSO packets that does > not hold for such packets generated by PF_PACKET. > > SCTP GSO introduced the GSO_BY_FRAGS mss value, so the code > takes a different path for SCTP packets generated by the SCTP stack. > > PF_PACKET does not necessarily set gso_size to GSO_BY_FRAGS, so > does not take the branch that requires list_skb to be non-zero here: > > if (unlikely(mss == GSO_BY_FRAGS)) { > len = list_skb->len; > } else { > len = head_skb->len - offset; > if (len > mss) > len = mss; > } > > hsize = skb_headlen(head_skb) - offset; > if (hsize < 0) > hsize = 0; > if (hsize > len || !sg) > hsize = len; > > if (!hsize && i >= nfrags && skb_headlen(list_skb) && > (skb_headlen(list_skb) == len || sg)) { > > Somewhat tangential, but any PF_PACKET socket can set this > magic gso_size value in its virtio_net_hdr, so if it is assumed to > be an SCTP GSO specific option, setting it for a TCP GSO packet > may also cause unexpected results. It seems virtio_net could use more sanity checks. When PACKET_VNET_HDR is used, it will end up calling: tpacket_rcv() { ... if (do_vnet) { if (virtio_net_hdr_from_skb(skb, h.raw + macoff - sizeof(struct virtio_net_hdr), vio_le(), true)) { spin_lock(&sk->sk_receive_queue.lock); goto drop_n_account; } } and virtio_net_hdr_from_skb does: if (skb_is_gso(skb)) { ... if (sinfo->gso_type & SKB_GSO_TCPV4) hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4; else if (sinfo->gso_type & SKB_GSO_TCPV6) hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6; else return -EINVAL; Meaning that any gso_type other than TCP would be rejected, but this SCTP one got through. Seems the header contains a sctp header, but the gso_type set was actually pointing to TCP (otherwise it would have been rejected). AFAICT if this packet had an ESP header, for example, it could have hit esp4_gso_segment. Can you please confirm this? I don't know of anywhere in the stack validating if the gso_type matches the header that actually is in there. The fix you mentioned is a good start, we want that one way or another, but I'm afraid this bug is bigger than sctp. Marcelo -- To unsubscribe from this list: send the line "unsubscribe linux-sctp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
