• Daniel Borkmann's avatar
    net: sctp: fix panic on duplicate ASCONF chunks · b69040d8
    Daniel Borkmann authored
    When receiving a e.g. semi-good formed connection scan in the
    form of ...
    
      -------------- INIT[ASCONF; ASCONF_ACK] ------------->
      <----------- INIT-ACK[ASCONF; ASCONF_ACK] ------------
      -------------------- COOKIE-ECHO -------------------->
      <-------------------- COOKIE-ACK ---------------------
      ---------------- ASCONF_a; ASCONF_b ----------------->
    
    ... where ASCONF_a equals ASCONF_b chunk (at least both serials
    need to be equal), we panic an SCTP server!
    
    The problem is that good-formed ASCONF chunks that we reply with
    ASCONF_ACK chunks are cached per serial. Thus, when we receive a
    same ASCONF chunk twice (e.g. through a lost ASCONF_ACK), we do
    not need to process them again on the server side (that was the
    idea, also proposed in the RFC). Instead, we know it was cached
    and we just resend the cached chunk instead. So far, so good.
    
    Where things get nasty is in SCTP's side effect interpreter, that
    is, sctp_cmd_interpreter():
    
    While incoming ASCONF_a (chunk = event_arg) is being marked
    !end_of_packet and !singleton, and we have an association context,
    we do not flush the outqueue the first time after processing the
    ASCONF_ACK singleton chunk via SCTP_CMD_REPLY. Instead, we keep it
    queued up, although we set local_cork to 1. Commit 2e3216cd
    changed the precedence, so that as long as we get bundled, incoming
    chunks we try possible bundling on outgoing queue as well. Before
    this commit, we would just flush the output queue.
    
    Now, while ASCONF_a's ASCONF_ACK sits in the corked outq, we
    continue to process the same ASCONF_b chunk from the packet. As
    we have cached the previous ASCONF_ACK, we find it, grab it and
    do another SCTP_CMD_REPLY command on it. So, effectively, we rip
    the chunk->list pointers and requeue the same ASCONF_ACK chunk
    another time. Since we process ASCONF_b, it's correctly marked
    with end_of_packet and we enforce an uncork, and thus flush, thus
    crashing the kernel.
    
    Fix it by testing if the ASCONF_ACK is currently pending and if
    that is the case, do not requeue it. When flushing the output
    queue we may relink the chunk for preparing an outgoing packet,
    but eventually unlink it when it's copied into the skb right
    before transmission.
    
    Joint work with Vlad Yasevich.
    
    Fixes: 2e3216cd
    
     ("sctp: Follow security requirement of responding with 1 packet")
    Signed-off-by: default avatarDaniel Borkmann <dborkman@redhat.com>
    Signed-off-by: default avatarVlad Yasevich <vyasevich@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    b69040d8
sctp.h 18.2 KB