Vulnerability: The sctp_sf_ootb function in net/sctp/sm_statefuns.c in the Linux kernel before 4.8.8 lacks chunk-length checking for the first chunk, which allows remote attackers to cause a denial of service (out-of-bounds slab access) or possibly have unspecified other impact via crafted SCTP data.
Vulnerability ID: CVE-2016-9555
CWE: CWE-125
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2016-11-28
File: net/sctp/sm_statefuns.c
Commit message: sctp: validate chunk len before actually using it

Andrey Konovalov reported that KASAN detected that SCTP was using a slab
beyond the boundaries. It was caused because when handling out of the
blue packets in function sctp_sf_ootb() it was checking the chunk len
only after already processing the first chunk, validating only for the
2nd and subsequent ones.

The fix is to just move the check upwards so it's also validated for the
1st chunk.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Commit author: Marcelo Ricardo Leitner

Vulnerable version (before fix):
 sctp_disposition_t sctp_sf_ootb(struct net *net, 			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg, 						  commands);  		/* Now that we know we at least have a chunk header, 		 * do things that are type appropriate. 		 */ sctp_disposition_t sctp_sf_ootb(struct net *net, 			} 		}  		/* Report violation if chunk len overflows */ 		ch_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length)); 		if (ch_end > skb_tail_pointer(skb)) 			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg, 						  commands);  		ch = (sctp_chunkhdr_t *) ch_end; 	} while (ch_end < skb_tail_pointer(skb)); 

Fixed version (after fix):
 sctp_disposition_t sctp_sf_ootb(struct net *net, 			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg, 						  commands);  		/* Report violation if chunk len overflows */ 		ch_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length)); 		if (ch_end > skb_tail_pointer(skb)) 			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg, 						  commands);  		/* Now that we know we at least have a chunk header, 		 * do things that are type appropriate. 		 */ sctp_disposition_t sctp_sf_ootb(struct net *net, 			} 		}  		ch = (sctp_chunkhdr_t *) ch_end; 	} while (ch_end < skb_tail_pointer(skb)); 

Patch:
patch: @@ -3422,6 +3422,12 @@ sctp_disposition_t sctp_sf_ootb(struct net *net,
 			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
 						  commands);
 
+		/* Report violation if chunk len overflows */
+		ch_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length));
+		if (ch_end > skb_tail_pointer(skb))
+			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
+						  commands);
+
 		/* Now that we know we at least have a chunk header,
 		 * do things that are type appropriate.
 		 */
@@ -3453,12 +3459,6 @@ sctp_disposition_t sctp_sf_ootb(struct net *net,
 			}
 		}
 
-		/* Report violation if chunk len overflows */
-		ch_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length));
-		if (ch_end > skb_tail_pointer(skb))
-			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
-						  commands);
-
 		ch = (sctp_chunkhdr_t *) ch_end;
 	} while (ch_end < skb_tail_pointer(skb));
path: net/sctp/sm_statefuns.c