[PATCH net] ipv4: better IP_MAX_MTU enforcement

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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 <eduma...@google.com>
---
 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)