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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>
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Eric Dumazet authored and davem330 committed May 22, 2017
1 parent 6d18c73 commit 232cd35
Showing 1 changed file with 8 additions and 7 deletions.
15 changes: 8 additions & 7 deletions net/ipv6/ip6_output.c
Expand Up @@ -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,
Expand Down Expand Up @@ -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;
Expand Down

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