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>

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index d4a31becbd25dda895d7391e1e65c2de237bf2a3..bf8a58a1c32d83a9605844075da5815be23a6bf1 100644 (file)
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1466,6 +1466,11 @@ 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,
@@ -1515,13 +1520,9 @@ alloc_new_skb:
                                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;