ipv6: fix out of bound writes in __ip6_append_data()

Orabug: 26575181
CVE: CVE-2017-9242

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>
(cherry picked from commit 232cd35d0804cc241eb887bb8d4d9b3b9881c64a)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	net/ipv6/ip6_output.c

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 6b96198cc5ebdd72e6801b9466fdb8409d06b0e6..a5c8290f3505eaf7d79e768227b1fe414273871f 100644 (file)
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1501,6 +1501,11 @@ alloc_new_skb:
                         */
                        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,
@@ -1553,15 +1558,9 @@ alloc_new_skb:
                                data += fraggap;
                                pskb_trim_unique(skb_prev, maxfraglen);
                        }
-                       copy = datalen - transhdrlen - fraggap;
-
-                       if (copy < 0) {
-                               err = -EINVAL;
-                               kfree_skb(skb);
-                               skb = NULL;
-                               dropreason = "invalid header";
-                               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;
                                dropreason = "could not get fragment";
                                kfree_skb(skb);