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    Subject[PATCH 4.12 14/99] ipv4: better IP_MAX_MTU enforcement
    Date
    4.12-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Eric Dumazet <edumazet@google.com>


    [ Upstream commit c780a049f9bf442314335372c9abc4548bfe3e44 ]

    While working on yet another syzkaller report, I found
    that our IP_MAX_MTU enforcements were not properly done.

    gcc seems to reload dev->mtu for min(dev->mtu, IP_MAX_MTU), and
    final result can be bigger than IP_MAX_MTU :/

    This is a problem because device mtu can be changed on other cpus or
    threads.

    While this patch does not fix the issue I am working on, it is
    probably worth addressing it.

    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    include/net/ip.h | 4 ++--
    net/ipv4/route.c | 2 +-
    2 files changed, 3 insertions(+), 3 deletions(-)

    --- a/include/net/ip.h
    +++ b/include/net/ip.h
    @@ -352,7 +352,7 @@ static inline unsigned int ip_dst_mtu_ma
    !forwarding)
    return dst_mtu(dst);

    - return min(dst->dev->mtu, IP_MAX_MTU);
    + return min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU);
    }

    static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
    @@ -364,7 +364,7 @@ static inline unsigned int ip_skb_dst_mt
    return ip_dst_mtu_maybe_forward(skb_dst(skb), forwarding);
    }

    - return min(skb_dst(skb)->dev->mtu, IP_MAX_MTU);
    + return min(READ_ONCE(skb_dst(skb)->dev->mtu), IP_MAX_MTU);
    }

    u32 ip_idents_reserve(u32 hash, int segs);
    --- a/net/ipv4/route.c
    +++ b/net/ipv4/route.c
    @@ -1268,7 +1268,7 @@ static unsigned int ipv4_mtu(const struc
    if (mtu)
    return mtu;

    - mtu = dst->dev->mtu;
    + mtu = READ_ONCE(dst->dev->mtu);

    if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {
    if (rt->rt_uses_gateway && mtu > 576)

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