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 ec0223ec48a9 ("net: sctp: fix sctp_sf_do_5_1D_ce to verify if we/peer is AUTH capable") and b14878ccb7fa ("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: 730fc3d05cd4 ("[SCTP]: Implete SCTP-AUTH parameter processing") Reported-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com> Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com> Cc: Vlad Yasevich <vyasevich@gmail.com> --- v1 -> v2, more notes: I've only updated the commit description for now, this bug seems clear to me that we would need to fix it; since RFC4895 mentions it explicitly that on collisions, we need to *update* these params accordingly as we would do so in RFC2960. So in other words, this can be explained by having an *inconsistency* when doing the update as auth_capable is *tightly coupled* with peer_random, peer_chunks, peer_hmacs and eventually the asoc_shared_key creation. For the rest, I went through the code and currently could not find where we could oops if we don't have the others for now. It needs more time and testing however. It's also not too clear from RFC2960/RFC4960 what needs to be carried over in addition: so we know "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." and we know that we need to update all AUTH related members, which we do *now*. In addition, we also need to fix AUTH + COOKIE_ECHO collisions, as they currently cannot be resolved properly into a handshake. diff --git a/net/sctp/associola.c b/net/sctp/associola.c index 9de23a2..06a9ee6 100644 --- a/net/sctp/associola.c +++ b/net/sctp/associola.c @@ -1097,6 +1097,7 @@ void sctp_assoc_update(struct sctp_association *asoc, asoc->c = new->c; asoc->peer.rwnd = new->peer.rwnd; asoc->peer.sack_needed = new->peer.sack_needed; + asoc->peer.auth_capable = new->peer.auth_capable; asoc->peer.i = new->peer.i; sctp_tsnmap_init(&asoc->peer.tsn_map, SCTP_TSN_MAP_INITIAL, asoc->peer.i.initial_tsn, GFP_ATOMIC);