213 lines
9.6 KiB
Diff
213 lines
9.6 KiB
Diff
Bugzilla: 1123696
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Upstream-status: Queued for 3.16
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From patchwork Tue Jul 22 13:22:45 2014
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Content-Type: text/plain; charset="utf-8"
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MIME-Version: 1.0
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Content-Transfer-Encoding: 7bit
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Subject: [net,v2] net: sctp: inherit auth_capable on INIT collisions
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From: Daniel Borkmann <dborkman@redhat.com>
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X-Patchwork-Id: 372475
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Message-Id: <1406035365-1154-1-git-send-email-dborkman@redhat.com>
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To: davem@davemloft.net
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Cc: jgunthorpe@obsidianresearch.com, netdev@vger.kernel.org,
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linux-sctp@vger.kernel.org, Vlad Yasevich <vyasevich@gmail.com>
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Date: Tue, 22 Jul 2014 15:22:45 +0200
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Jason reported an oops caused by SCTP on his ARM machine with
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SCTP authentication enabled:
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Internal error: Oops: 17 [#1] ARM
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CPU: 0 PID: 104 Comm: sctp-test Not tainted 3.13.0-68744-g3632f30c9b20-dirty #1
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task: c6eefa40 ti: c6f52000 task.ti: c6f52000
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PC is at sctp_auth_calculate_hmac+0xc4/0x10c
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LR is at sg_init_table+0x20/0x38
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pc : [<c024bb80>] lr : [<c00f32dc>] psr: 40000013
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sp : c6f538e8 ip : 00000000 fp : c6f53924
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r10: c6f50d80 r9 : 00000000 r8 : 00010000
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r7 : 00000000 r6 : c7be4000 r5 : 00000000 r4 : c6f56254
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r3 : c00c8170 r2 : 00000001 r1 : 00000008 r0 : c6f1e660
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Flags: nZcv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
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Control: 0005397f Table: 06f28000 DAC: 00000015
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Process sctp-test (pid: 104, stack limit = 0xc6f521c0)
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Stack: (0xc6f538e8 to 0xc6f54000)
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[...]
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Backtrace:
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[<c024babc>] (sctp_auth_calculate_hmac+0x0/0x10c) from [<c0249af8>] (sctp_packet_transmit+0x33c/0x5c8)
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[<c02497bc>] (sctp_packet_transmit+0x0/0x5c8) from [<c023e96c>] (sctp_outq_flush+0x7fc/0x844)
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[<c023e170>] (sctp_outq_flush+0x0/0x844) from [<c023ef78>] (sctp_outq_uncork+0x24/0x28)
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[<c023ef54>] (sctp_outq_uncork+0x0/0x28) from [<c0234364>] (sctp_side_effects+0x1134/0x1220)
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[<c0233230>] (sctp_side_effects+0x0/0x1220) from [<c02330b0>] (sctp_do_sm+0xac/0xd4)
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[<c0233004>] (sctp_do_sm+0x0/0xd4) from [<c023675c>] (sctp_assoc_bh_rcv+0x118/0x160)
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[<c0236644>] (sctp_assoc_bh_rcv+0x0/0x160) from [<c023d5bc>] (sctp_inq_push+0x6c/0x74)
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[<c023d550>] (sctp_inq_push+0x0/0x74) from [<c024a6b0>] (sctp_rcv+0x7d8/0x888)
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While we already had various kind of bugs in that area
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ec0223ec48a9 ("net: sctp: fix sctp_sf_do_5_1D_ce to verify if
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we/peer is AUTH capable") and b14878ccb7fa ("net: sctp: cache
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auth_enable per endpoint"), this one is a bit of a different
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kind.
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Giving a bit more background on why SCTP authentication is
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needed can be found in RFC4895:
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SCTP uses 32-bit verification tags to protect itself against
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blind attackers. These values are not changed during the
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lifetime of an SCTP association.
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Looking at new SCTP extensions, there is the need to have a
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method of proving that an SCTP chunk(s) was really sent by
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the original peer that started the association and not by a
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malicious attacker.
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To cause this bug, we're triggering an INIT collision between
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peers; normal SCTP handshake where both sides intent to
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authenticate packets contains RANDOM; CHUNKS; HMAC-ALGO
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parameters that are being negotiated among peers:
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---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
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<------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
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-------------------- COOKIE-ECHO -------------------->
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<-------------------- COOKIE-ACK ---------------------
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RFC4895 says that each endpoint therefore knows its own random
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number and the peer's random number *after* the association
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has been established. The local and peer's random number along
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with the shared key are then part of the secret used for
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calculating the HMAC in the AUTH chunk.
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Now, in our scenario, we have 2 threads with 1 non-blocking
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SEQ_PACKET socket each, setting up common shared SCTP_AUTH_KEY
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and SCTP_AUTH_ACTIVE_KEY properly, and each of them calling
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sctp_bindx(3), listen(2) and connect(2) against each other,
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thus the handshake looks similar to this, e.g.:
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---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
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<------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
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<--------- INIT[RANDOM; CHUNKS; HMAC-ALGO] -----------
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-------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] -------->
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...
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Since such collisions can also happen with verification tags,
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the RFC4895 for AUTH rather vaguely says under section 6.1:
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In case of INIT collision, the rules governing the handling
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of this Random Number follow the same pattern as those for
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the Verification Tag, as explained in Section 5.2.4 of
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RFC 2960 [5]. Therefore, each endpoint knows its own Random
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Number and the peer's Random Number after the association
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has been established.
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In RFC2960, section 5.2.4, we're eventually hitting Action B:
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B) In this case, both sides may be attempting to start an
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association at about the same time but the peer endpoint
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started its INIT after responding to the local endpoint's
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INIT. Thus it may have picked a new Verification Tag not
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being aware of the previous Tag it had sent this endpoint.
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The endpoint should stay in or enter the ESTABLISHED
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state but it MUST update its peer's Verification Tag from
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the State Cookie, stop any init or cookie timers that may
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running and send a COOKIE ACK.
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In other words, the handling of the Random parameter is the
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same as behavior for the Verification Tag as described in
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Action B of section 5.2.4.
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Looking at the code, we exactly hit the sctp_sf_do_dupcook_b()
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case which triggers an SCTP_CMD_UPDATE_ASSOC command to the
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side effect interpreter, and in fact it properly copies over
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peer_{random, hmacs, chunks} parameters from the newly created
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association to update the existing one.
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Also, the old asoc_shared_key is being released and based on
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the new params, sctp_auth_asoc_init_active_key() updated.
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However, the issue observed in this case is that the previous
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asoc->peer.auth_capable was 0, and has *not* been updated, so
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that instead of creating a new secret, we're doing an early
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return from the function sctp_auth_asoc_init_active_key()
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leaving asoc->asoc_shared_key as NULL. However, we now have to
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authenticate chunks from the updated chunk list (e.g. COOKIE-ACK).
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That in fact causes the server side when responding with ...
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<------------------ AUTH; COOKIE-ACK -----------------
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... to trigger a NULL pointer dereference, since in
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sctp_packet_transmit(), it discovers that an AUTH chunk is
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being queued for xmit, and thus it calls sctp_auth_calculate_hmac().
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Since the asoc->active_key_id is still inherited from the
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endpoint, and the same as encoded into the chunk, it uses
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asoc->asoc_shared_key, which is still NULL, as an asoc_key
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and dereferences it in ...
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crypto_hash_setkey(desc.tfm, &asoc_key->data[0], asoc_key->len)
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... causing an oops. All this happens because sctp_make_cookie_ack()
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called with the *new* association has the peer.auth_capable=1
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and therefore marks the chunk with auth=1 after checking
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sctp_auth_send_cid(), but it is *actually* sent later on over
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the then *updated* association's transport that didn't initialize
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its shared key due to peer.auth_capable=0. Since control chunks
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in that case are not sent by the temporary association which
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are scheduled for deletion, they are issued for xmit via
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SCTP_CMD_REPLY in the interpreter with the context of the
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*updated* association. peer.auth_capable was 0 in the updated
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association (which went from COOKIE_WAIT into ESTABLISHED state),
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since all previous processing that performed sctp_process_init()
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was being done on temporary associations, that we eventually
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throw away each time.
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The correct fix is to update to the new peer.auth_capable
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value as well in the collision case via sctp_assoc_update(),
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so that in case the collision migrated from 0 -> 1,
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sctp_auth_asoc_init_active_key() can properly recalculate
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the secret. This therefore fixes the observed server panic.
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Fixes: 730fc3d05cd4 ("[SCTP]: Implete SCTP-AUTH parameter processing")
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Reported-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
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Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
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Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
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Cc: Vlad Yasevich <vyasevich@gmail.com>
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Acked-by: Vlad Yasevich <vyasevich@gmail.com>
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---
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v1 -> v2, more notes:
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I've only updated the commit description for now, this bug seems
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clear to me that we would need to fix it; since RFC4895 mentions
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it explicitly that on collisions, we need to *update* these params
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accordingly as we would do so in RFC2960. So in other words, this
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can be explained by having an *inconsistency* when doing the update
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as auth_capable is *tightly coupled* with peer_random, peer_chunks,
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peer_hmacs and eventually the asoc_shared_key creation.
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For the rest, I went through the code and currently could not
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find where we could oops if we don't have the others for now. It
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needs more time and testing however. It's also not too clear from
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RFC2960/RFC4960 what needs to be carried over in addition: so we
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know "The endpoint should stay in or enter the ESTABLISHED state
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but it MUST update its peer's Verification Tag from the State
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Cookie, stop any init or cookie timers that may running and send
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a COOKIE ACK." and we know that we need to update all AUTH related
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members, which we do *now*.
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In addition, we also need to fix AUTH + COOKIE_ECHO collisions,
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as they currently cannot be resolved properly into a handshake.
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net/sctp/associola.c | 1 +
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1 file changed, 1 insertion(+)
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diff --git a/net/sctp/associola.c b/net/sctp/associola.c
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index 9de23a2..06a9ee6 100644
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--- a/net/sctp/associola.c
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+++ b/net/sctp/associola.c
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@@ -1097,6 +1097,7 @@ void sctp_assoc_update(struct sctp_association *asoc,
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asoc->c = new->c;
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asoc->peer.rwnd = new->peer.rwnd;
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asoc->peer.sack_needed = new->peer.sack_needed;
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+ asoc->peer.auth_capable = new->peer.auth_capable;
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asoc->peer.i = new->peer.i;
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sctp_tsnmap_init(&asoc->peer.tsn_map, SCTP_TSN_MAP_INITIAL,
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asoc->peer.i.initial_tsn, GFP_ATOMIC);
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