• Daniel Borkmann's avatar
    net: sctp: inherit auth_capable on INIT collisions · 1be9a950
    Daniel Borkmann authored
    Jason reported an oops caused by SCTP on his ARM machine with
    SCTP authentication enabled:
    
    Internal error: Oops: 17 [#1] ARM
    CPU: 0 PID: 104 Comm: sctp-test Not tainted 3.13.0-68744-g3632f30c9b20-dirty #1
    task: c6eefa40 ti: c6f52000 task.ti: c6f52000
    PC is at sctp_auth_calculate_hmac+0xc4/0x10c
    LR is at sg_init_table+0x20/0x38
    pc : [<c024bb80>]    lr : [<c00f32dc>]    psr: 40000013
    sp : c6f538e8  ip : 00000000  fp : c6f53924
    r10: c6f50d80  r9 : 00000000  r8 : 00010000
    r7 : 00000000  r6 : c7be4000  r5 : 00000000  r4 : c6f56254
    r3 : c00c8170  r2 : 00000001  r1 : 00000008  r0 : c6f1e660
    Flags: nZcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
    Control: 0005397f  Table: 06f28000  DAC: 00000015
    Process sctp-test (pid: 104, stack limit = 0xc6f521c0)
    Stack: (0xc6f538e8 to 0xc6f54000)
    [...]
    Backtrace:
    [<c024babc>] (sctp_auth_calculate_hmac+0x0/0x10c) from [<c0249af8>] (sctp_packet_transmit+0x33c/0x5c8)
    [<c02497bc>] (sctp_packet_transmit+0x0/0x5c8) from [<c023e96c>] (sctp_outq_flush+0x7fc/0x844)
    [<c023e170>] (sctp_outq_flush+0x0/0x844) from [<c023ef78>] (sctp_outq_uncork+0x24/0x28)
    [<c023ef54>] (sctp_outq_uncork+0x0/0x28) from [<c0234364>] (sctp_side_effects+0x1134/0x1220)
    [<c0233230>] (sctp_side_effects+0x0/0x1220) from [<c02330b0>] (sctp_do_sm+0xac/0xd4)
    [<c0233004>] (sctp_do_sm+0x0/0xd4) from [<c023675c>] (sctp_assoc_bh_rcv+0x118/0x160)
    [<c0236644>] (sctp_assoc_bh_rcv+0x0/0x160) from [<c023d5bc>] (sctp_inq_push+0x6c/0x74)
    [<c023d550>] (sctp_inq_push+0x0/0x74) from [<c024a6b0>] (sctp_rcv+0x7d8/0x888)
    
    While we already had various kind of bugs in that area
    ec0223ec ("net: sctp: fix sctp_sf_do_5_1D_ce to verify if
    we/peer is AUTH capable") and b14878cc ("net: sctp: cache
    auth_enable per endpoint"), this one is a bit of a different
    kind.
    
    Giving a bit more background on why SCTP authentication is
    needed can be found in RFC4895:
    
      SCTP uses 32-bit verification tags to protect itself against
      blind attackers. These values are not changed during the
      lifetime of an SCTP association.
    
      Looking at new SCTP extensions, there is the need to have a
      method of proving that an SCTP chunk(s) was really sent by
      the original peer that started the association and not by a
      malicious attacker.
    
    To cause this bug, we're triggering an INIT collision between
    peers; normal SCTP handshake where both sides intent to
    authenticate packets contains RANDOM; CHUNKS; HMAC-ALGO
    parameters that are being negotiated among peers:
    
      ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
      <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
      -------------------- COOKIE-ECHO -------------------->
      <-------------------- COOKIE-ACK ---------------------
    
    RFC4895 says that each endpoint therefore knows its own random
    number and the peer's random number *after* the association
    has been established. The local and peer's random number along
    with the shared key are then part of the secret used for
    calculating the HMAC in the AUTH chunk.
    
    Now, in our scenario, we have 2 threads with 1 non-blocking
    SEQ_PACKET socket each, setting up common shared SCTP_AUTH_KEY
    and SCTP_AUTH_ACTIVE_KEY properly, and each of them calling
    sctp_bindx(3), listen(2) and connect(2) against each other,
    thus the handshake looks similar to this, e.g.:
    
      ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
      <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
      <--------- INIT[RANDOM; CHUNKS; HMAC-ALGO] -----------
      -------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] -------->
      ...
    
    Since such collisions can also happen with verification tags,
    the RFC4895 for AUTH rather vaguely says under section 6.1:
    
      In case of INIT collision, the rules governing the handling
      of this Random Number follow the same pattern as those for
      the Verification Tag, as explained in Section 5.2.4 of
      RFC 2960 [5]. Therefore, each endpoint knows its own Random
      Number and the peer's Random Number after the association
      has been established.
    
    In RFC2960, section 5.2.4, we're eventually hitting Action B:
    
      B) In this case, both sides may be attempting to start an
         association at about the same time but the peer endpoint
         started its INIT after responding to the local endpoint's
         INIT. Thus it may have picked a new Verification Tag not
         being aware of the previous Tag it had sent this endpoint.
         The endpoint should stay in or enter the ESTABLISHED
         state but it MUST update its peer's Verification Tag from
         the State Cookie, stop any init or cookie timers that may
         running and send a COOKIE ACK.
    
    In other words, the handling of the Random parameter is the
    same as behavior for the Verification Tag as described in
    Action B of section 5.2.4.
    
    Looking at the code, we exactly hit the sctp_sf_do_dupcook_b()
    case which triggers an SCTP_CMD_UPDATE_ASSOC command to the
    side effect interpreter, and in fact it properly copies over
    peer_{random, hmacs, chunks} parameters from the newly created
    association to update the existing one.
    
    Also, the old asoc_shared_key is being released and based on
    the new params, sctp_auth_asoc_init_active_key() updated.
    However, the issue observed in this case is that the previous
    asoc->peer.auth_capable was 0, and has *not* been updated, so
    that instead of creating a new secret, we're doing an early
    return from the function sctp_auth_asoc_init_active_key()
    leaving asoc->asoc_shared_key as NULL. However, we now have to
    authenticate chunks from the updated chunk list (e.g. COOKIE-ACK).
    
    That in fact causes the server side when responding with ...
    
      <------------------ AUTH; COOKIE-ACK -----------------
    
    ... to trigger a NULL pointer dereference, since in
    sctp_packet_transmit(), it discovers that an AUTH chunk is
    being queued for xmit, and thus it calls sctp_auth_calculate_hmac().
    
    Since the asoc->active_key_id is still inherited from the
    endpoint, and the same as encoded into the chunk, it uses
    asoc->asoc_shared_key, which is still NULL, as an asoc_key
    and dereferences it in ...
    
      crypto_hash_setkey(desc.tfm, &asoc_key->data[0], asoc_key->len)
    
    ... causing an oops. All this happens because sctp_make_cookie_ack()
    called with the *new* association has the peer.auth_capable=1
    and therefore marks the chunk with auth=1 after checking
    sctp_auth_send_cid(), but it is *actually* sent later on over
    the then *updated* association's transport that didn't initialize
    its shared key due to peer.auth_capable=0. Since control chunks
    in that case are not sent by the temporary association which
    are scheduled for deletion, they are issued for xmit via
    SCTP_CMD_REPLY in the interpreter with the context of the
    *updated* association. peer.auth_capable was 0 in the updated
    association (which went from COOKIE_WAIT into ESTABLISHED state),
    since all previous processing that performed sctp_process_init()
    was being done on temporary associations, that we eventually
    throw away each time.
    
    The correct fix is to update to the new peer.auth_capable
    value as well in the collision case via sctp_assoc_update(),
    so that in case the collision migrated from 0 -> 1,
    sctp_auth_asoc_init_active_key() can properly recalculate
    the secret. This therefore fixes the observed server panic.
    
    Fixes: 730fc3d0
    
     ("[SCTP]: Implete SCTP-AUTH parameter processing")
    Reported-by: default avatarJason Gunthorpe <jgunthorpe@obsidianresearch.com>
    Signed-off-by: default avatarDaniel Borkmann <dborkman@redhat.com>
    Tested-by: default avatarJason Gunthorpe <jgunthorpe@obsidianresearch.com>
    Cc: Vlad Yasevich <vyasevich@gmail.com>
    Acked-by: default avatarVlad Yasevich <vyasevich@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    1be9a950
associola.c 48.2 KB