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    Subject[PATCH 3.10 118/250] ipv4: keep skb->dst around in presence of IP options
    Date
    From: Eric Dumazet <edumazet@google.com>

    commit 34b2cef20f19c87999fff3da4071e66937db9644 upstream.

    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: d826eb14ecef ("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>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    ---
    net/ipv4/ip_sockglue.c | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
    index f660314..3d009e1 100644
    --- a/net/ipv4/ip_sockglue.c
    +++ b/net/ipv4/ip_sockglue.c
    @@ -1042,7 +1042,14 @@ void ipv4_pktinfo_prepare(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,
    --
    2.8.0.rc2.1.gbe9624a
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