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ipv4: better IP_MAX_MTU enforcement

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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>
This commit is contained in:
Eric Dumazet 2017-08-16 11:09:12 -07:00 committed by David S. Miller
parent 81fbfe8ada
commit c780a049f9
2 changed files with 3 additions and 3 deletions
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4
include/net/ip.h
View File

@ -352,7 +352,7 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
!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_mtu(struct sock *sk,
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);

2
net/ipv4/route.c
View File

@ -1267,7 +1267,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst)
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)