Patch "ipv4: better IP_MAX_MTU enforcement" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ipv4: better IP_MAX_MTU enforcement

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ipv4-better-ip_max_mtu-enforcement.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Eric Dumazet <[email protected]>
Date: Wed, 16 Aug 2017 11:09:12 -0700
Subject: ipv4: better IP_MAX_MTU enforcement

From: Eric Dumazet <[email protected]>

[ 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.

---
 include/net/ip.h |    4 ++--
 net/ipv4/route.c |    2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

@@ -314,7 +314,7 @@ 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(const struct sk_buff *skb)

@@ -327,7 +327,7 @@ 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);

@@ -1241,7 +1241,7 @@ 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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/ipv4-fix-null-dereference-in-free_fib_info_rcu.patch queue-4.4/tcp-when-rearming-rto-if-rto-time-is-in-past-then-fire-rto-asap.patch queue-4.4/ipv4-better-ip_max_mtu-enforcement.patch queue-4.4/dccp-defer-ccid_hc_tx_delete-at-dismantle-time.patch queue-4.4/tipc-fix-use-after-free.patch queue-4.4/af_key-do-not-use-gfp_kernel-in-atomic-contexts.patch queue-4.4/ipv6-repair-fib6-tree-in-failure-case.patch queue-4.4/dccp-purge-write-queue-in-dccp_destroy_sock.patch queue-4.4/ipv6-reset-fn-rr_ptr-when-replacing-route.patch queue-4.4/net_sched-sfq-update-hierarchical-backlog-when-drop-packet.patch