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ipv4: keep skb->dst around in presence of IP options

Andrey Konovalov got crashes in __ip_options_echo() when a NULL skb->dst
is accessed.

ipv4_pktinfo_prepare() should not drop the dst if (evil) IP options
are present.

We could refine the test to the presence of ts_needtime or srr,
but IP options are not often used, so let's be conservative.

Thanks to syzkaller team for finding this bug.

Fixes: d826eb1 ("ipv4: PKTINFO doesnt need dst reference")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Eric Dumazet authored and davem330 committed Feb 4, 2017
1 parent 5fa8bbd commit 34b2cef20f19c87999fff3da4071e66937db9644
Showing with 8 additions and 1 deletion.
  1. +8 −1 net/ipv4/ip_sockglue.c
@@ -1238,7 +1238,14 @@ void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
pktinfo->ipi_ifindex = 0;
pktinfo->ipi_spec_dst.s_addr = 0;
}
skb_dst_drop(skb);
/* We need to keep the dst for __ip_options_echo()
* We could restrict the test to opt.ts_needtime || opt.srr,
* but the following is good enough as IP options are not often used.
*/
if (unlikely(IPCB(skb)->opt.optlen))
skb_dst_force(skb);
else
skb_dst_drop(skb);
}

int ip_setsockopt(struct sock *sk, int level,

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