Vulnerability: The sctp_process_param function in net/sctp/sm_make_chunk.c in the SCTP implementation in the Linux kernel before 3.17.4, when ASCONF is used, allows remote attackers to cause a denial of service (NULL pointer dereference and system crash) via a malformed INIT chunk.
Vulnerability ID: CVE-2014-7841
CWE: CWE-399
Severity score: 2.9
Project: https://github.com/torvalds/linux
Published: 2014-11-30
File: net/sctp/sm_make_chunk.c
Commit message: net: sctp: fix NULL pointer dereference in af->from_addr_param on malformed packet

An SCTP server doing ASCONF will panic on malformed INIT ping-of-death
in the form of:

  ------------ INIT[PARAM: SET_PRIMARY_IP] ------------>

While the INIT chunk parameter verification dissects through many things
in order to detect malformed input, it misses to actually check parameters
inside of parameters. E.g. RFC5061, section 4.2.4 proposes a 'set primary
IP address' parameter in ASCONF, which has as a subparameter an address
parameter.

So an attacker may send a parameter type other than SCTP_PARAM_IPV4_ADDRESS
or SCTP_PARAM_IPV6_ADDRESS, param_type2af() will subsequently return 0
and thus sctp_get_af_specific() returns NULL, too, which we then happily
dereference unconditionally through af->from_addr_param().

The trace for the log:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000078
IP: [<ffffffffa01e9c62>] sctp_process_init+0x492/0x990 [sctp]
PGD 0
Oops: 0000 [#1] SMP
[...]
Pid: 0, comm: swapper Not tainted 2.6.32-504.el6.x86_64 #1 Bochs Bochs
RIP: 0010:[<ffffffffa01e9c62>]  [<ffffffffa01e9c62>] sctp_process_init+0x492/0x990 [sctp]
[...]
Call Trace:
 <IRQ>
 [<ffffffffa01f2add>] ? sctp_bind_addr_copy+0x5d/0xe0 [sctp]
 [<ffffffffa01e1fcb>] sctp_sf_do_5_1B_init+0x21b/0x340 [sctp]
 [<ffffffffa01e3751>] sctp_do_sm+0x71/0x1210 [sctp]
 [<ffffffffa01e5c09>] ? sctp_endpoint_lookup_assoc+0xc9/0xf0 [sctp]
 [<ffffffffa01e61f6>] sctp_endpoint_bh_rcv+0x116/0x230 [sctp]
 [<ffffffffa01ee986>] sctp_inq_push+0x56/0x80 [sctp]
 [<ffffffffa01fcc42>] sctp_rcv+0x982/0xa10 [sctp]
 [<ffffffffa01d5123>] ? ipt_local_in_hook+0x23/0x28 [iptable_filter]
 [<ffffffff8148bdc9>] ? nf_iterate+0x69/0xb0
 [<ffffffff81496d10>] ? ip_local_deliver_finish+0x0/0x2d0
 [<ffffffff8148bf86>] ? nf_hook_slow+0x76/0x120
 [<ffffffff81496d10>] ? ip_local_deliver_finish+0x0/0x2d0
[...]

A minimal way to address this is to check for NULL as we do on all
other such occasions where we know sctp_get_af_specific() could
possibly return with NULL.

Fixes: d6de3097592b ("[SCTP]: Add the handling of "Set Primary IP Address" parameter to INIT")
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Commit author: Daniel Borkmann

Vulnerable version (before fix):
 static int sctp_process_param(struct sctp_association *asoc, 		addr_param = param.v + sizeof(sctp_addip_param_t);  		af = sctp_get_af_specific(param_type2af(param.p->type)); 		af->from_addr_param(&addr, addr_param, 				    htons(asoc->peer.port), 0); 

Fixed version (after fix):
 static int sctp_process_param(struct sctp_association *asoc, 		addr_param = param.v + sizeof(sctp_addip_param_t);  		af = sctp_get_af_specific(param_type2af(param.p->type)); 		if (af == NULL) 			break;  		af->from_addr_param(&addr, addr_param, 				    htons(asoc->peer.port), 0); 

Patch:
patch: @@ -2609,6 +2609,9 @@ static int sctp_process_param(struct sctp_association *asoc,
 		addr_param = param.v + sizeof(sctp_addip_param_t);
 
 		af = sctp_get_af_specific(param_type2af(param.p->type));
+		if (af == NULL)
+			break;
+
 		af->from_addr_param(&addr, addr_param,
 				    htons(asoc->peer.port), 0);
path: net/sctp/sm_make_chunk.c