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net: sctp: fix sctp_sf_do_5_1D_ce to verify if we/peer is AUTH capable
RFC4895 introduced AUTH chunks for SCTP; during the SCTP handshake RANDOM; CHUNKS; HMAC-ALGO are negotiated (CHUNKS being optional though): ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ----------> <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] --------- -------------------- COOKIE-ECHO --------------------> <-------------------- COOKIE-ACK --------------------- A special case is when an endpoint requires COOKIE-ECHO chunks to be authenticated: ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ----------> <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] --------- ------------------ AUTH; COOKIE-ECHO ----------------> <-------------------- COOKIE-ACK --------------------- RFC4895, section 6.3. Receiving Authenticated Chunks says: The receiver MUST use the HMAC algorithm indicated in the HMAC Identifier field. If this algorithm was not specified by the receiver in the HMAC-ALGO parameter in the INIT or INIT-ACK chunk during association setup, the AUTH chunk and all the chunks after it MUST be discarded and an ERROR chunk SHOULD be sent with the error cause defined in Section 4.1. [...] If no endpoint pair shared key has been configured for that Shared Key Identifier, all authenticated chunks MUST be silently discarded. [...] When an endpoint requires COOKIE-ECHO chunks to be authenticated, some special procedures have to be followed because the reception of a COOKIE-ECHO chunk might result in the creation of an SCTP association. If a packet arrives containing an AUTH chunk as a first chunk, a COOKIE-ECHO chunk as the second chunk, and possibly more chunks after them, and the receiver does not have an STCB for that packet, then authentication is based on the contents of the COOKIE-ECHO chunk. In this situation, the receiver MUST authenticate the chunks in the packet by using the RANDOM parameters, CHUNKS parameters and HMAC_ALGO parameters obtained from the COOKIE-ECHO chunk, and possibly a local shared secret as inputs to the authentication procedure specified in Section 6.3. If authentication fails, then the packet is discarded. If the authentication is successful, the COOKIE-ECHO and all the chunks after the COOKIE-ECHO MUST be processed. If the receiver has an STCB, it MUST process the AUTH chunk as described above using the STCB from the existing association to authenticate the COOKIE-ECHO chunk and all the chunks after it. [...] Commit bbd0d59 introduced the possibility to receive and verification of AUTH chunk, including the edge case for authenticated COOKIE-ECHO. On reception of COOKIE-ECHO, the function sctp_sf_do_5_1D_ce() handles processing, unpacks and creates a new association if it passed sanity checks and also tests for authentication chunks being present. After a new association has been processed, it invokes sctp_process_init() on the new association and walks through the parameter list it received from the INIT chunk. It checks SCTP_PARAM_RANDOM, SCTP_PARAM_HMAC_ALGO and SCTP_PARAM_CHUNKS, and copies them into asoc->peer meta data (peer_random, peer_hmacs, peer_chunks) in case sysctl -w net.sctp.auth_enable=1 is set. If in INIT's SCTP_PARAM_SUPPORTED_EXT parameter SCTP_CID_AUTH is set, peer_random != NULL and peer_hmacs != NULL the peer is to be assumed asoc->peer.auth_capable=1, in any other case asoc->peer.auth_capable=0. Now, if in sctp_sf_do_5_1D_ce() chunk->auth_chunk is available, we set up a fake auth chunk and pass that on to sctp_sf_authenticate(), which at latest in sctp_auth_calculate_hmac() reliably dereferences a NULL pointer at position 0..0008 when setting up the crypto key in crypto_hash_setkey() by using asoc->asoc_shared_key that is NULL as condition key_id == asoc->active_key_id is true if the AUTH chunk was injected correctly from remote. This happens no matter what net.sctp.auth_enable sysctl says. The fix is to check for net->sctp.auth_enable and for asoc->peer.auth_capable before doing any operations like sctp_sf_authenticate() as no key is activated in sctp_auth_asoc_init_active_key() for each case. Now as RFC4895 section 6.3 states that if the used HMAC-ALGO passed from the INIT chunk was not used in the AUTH chunk, we SHOULD send an error; however in this case it would be better to just silently discard such a maliciously prepared handshake as we didn't even receive a parameter at all. Also, as our endpoint has no shared key configured, section 6.3 says that MUST silently discard, which we are doing from now onwards. Before calling sctp_sf_pdiscard(), we need not only to free the association, but also the chunk->auth_chunk skb, as commit bbd0d59 created a skb clone in that case. I have tested this locally by using netfilter's nfqueue and re-injecting packets into the local stack after maliciously modifying the INIT chunk (removing RANDOM; HMAC-ALGO param) and the SCTP packet containing the COOKIE_ECHO (injecting AUTH chunk before COOKIE_ECHO). Fixed with this patch applied. Fixes: bbd0d59 ("[SCTP]: Implement the receive and verification of AUTH chunk") Signed-off-by: Daniel Borkmann <email@example.com> Cc: Vlad Yasevich <firstname.lastname@example.org> Cc: Neil Horman <email@example.com> Acked-by: Vlad Yasevich <firstname.lastname@example.org> Signed-off-by: David S. Miller <email@example.com>
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