Vulnerability: The __ip6_append_data function in net/ipv6/ip6_output.c in the Linux kernel through 4.11.3 is too late in checking whether an overwrite of an skb data structure may occur, which allows local users to cause a denial of service (system crash) via crafted system calls.
Vulnerability ID: CVE-2017-9242
CWE: CWE-20
Severity score: 6.9
Project: https://github.com/torvalds/linux
Published: 2017-05-27
File: net/ipv6/ip6_output.c
Commit message: ipv6: fix out of bound writes in __ip6_append_data()

Andrey Konovalov and idaifish@gmail.com reported crashes caused by
one skb shared_info being overwritten from __ip6_append_data()

Andrey program lead to following state :

copy -4200 datalen 2000 fraglen 2040
maxfraglen 2040 alloclen 2048 transhdrlen 0 offset 0 fraggap 6200

The skb_copy_and_csum_bits(skb_prev, maxfraglen, data + transhdrlen,
fraggap, 0); is overwriting skb->head and skb_shared_info

Since we apparently detect this rare condition too late, move the
code earlier to even avoid allocating skb and risking crashes.

Once again, many thanks to Andrey and syzkaller team.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Reported-by: <idaifish@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Commit author: Eric Dumazet

Vulnerable version (before fix):
 static int __ip6_append_data(struct sock *sk, 			 */ 			alloclen += sizeof(struct frag_hdr);  			if (transhdrlen) { 				skb = sock_alloc_send_skb(sk, 						alloclen + hh_len, static int __ip6_append_data(struct sock *sk, 				data += fraggap; 				pskb_trim_unique(skb_prev, maxfraglen); 			} 			copy = datalen - transhdrlen - fraggap;  			if (copy < 0) { 				err = -EINVAL; 				kfree_skb(skb); 				goto error; 			} else if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, fraggap, skb) < 0) { 				err = -EFAULT; 				kfree_skb(skb); 				goto error;

Fixed version (after fix):
 static int __ip6_append_data(struct sock *sk, 			 */ 			alloclen += sizeof(struct frag_hdr);  			copy = datalen - transhdrlen - fraggap; 			if (copy < 0) { 				err = -EINVAL; 				goto error; 			} 			if (transhdrlen) { 				skb = sock_alloc_send_skb(sk, 						alloclen + hh_len, static int __ip6_append_data(struct sock *sk, 				data += fraggap; 				pskb_trim_unique(skb_prev, maxfraglen); 			} 			if (copy > 0 && 			    getfrag(from, data + transhdrlen, offset, 				    copy, fraggap, skb) < 0) { 				err = -EFAULT; 				kfree_skb(skb); 				goto error;

Patch:
patch: @@ -1466,6 +1466,11 @@ static int __ip6_append_data(struct sock *sk,
 			 */
 			alloclen += sizeof(struct frag_hdr);
 
+			copy = datalen - transhdrlen - fraggap;
+			if (copy < 0) {
+				err = -EINVAL;
+				goto error;
+			}
 			if (transhdrlen) {
 				skb = sock_alloc_send_skb(sk,
 						alloclen + hh_len,
@@ -1515,13 +1520,9 @@ static int __ip6_append_data(struct sock *sk,
 				data += fraggap;
 				pskb_trim_unique(skb_prev, maxfraglen);
 			}
-			copy = datalen - transhdrlen - fraggap;
-
-			if (copy < 0) {
-				err = -EINVAL;
-				kfree_skb(skb);
-				goto error;
-			} else if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, fraggap, skb) < 0) {
+			if (copy > 0 &&
+			    getfrag(from, data + transhdrlen, offset,
+				    copy, fraggap, skb) < 0) {
 				err = -EFAULT;
 				kfree_skb(skb);
 				goto error;
path: net/ipv6/ip6_output.c