Commit c780a049 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

ipv4: better IP_MAX_MTU enforcement 



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: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 81fbfe8a

include/net/ip.h

+2 −2
Original line number Original line Diff line number Diff line
@@ -352,7 +352,7 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst, 
	    !forwarding)

	    !forwarding)

		return dst_mtu(dst);

		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,

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 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);

u32 ip_idents_reserve(u32 hash, int segs);

net/ipv4/route.c

+1 −1
Original line number Original line Diff line number Diff line
@@ -1267,7 +1267,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst) 
	if (mtu)

	if (mtu)

		return mtu;

		return mtu;





	mtu = dst->dev->mtu;

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





	if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {

	if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {

		if (rt->rt_uses_gateway && mtu > 576)

		if (rt->rt_uses_gateway && mtu > 576)

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