From c780a049f9bf442314335372c9abc4548bfe3e44 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 16 Aug 2017 11:09:12 -0700 Subject: [PATCH] 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: Eric Dumazet Signed-off-by: David S. Miller --- include/net/ip.h | 4 ++-- net/ipv4/route.c | 2 +- 2 files changed, 3 insertions(+), 3 deletions(-) diff --git a/include/net/ip.h b/include/net/ip.h index 821cedcc8e73..0cf7f5a65fe6 100644 --- a/include/net/ip.h +++ b/include/net/ip.h @@ -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); diff --git a/net/ipv4/route.c b/net/ipv4/route.c index 7effa62beed3..fe877a4a72b1 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -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) -- 2.20.1