[PATCH 4.12 14/99] ipv4: better IP_MAX_MTU enforcement

Greg Kroah-Hartman
2017-08-28T09:24:09Z

From: Eric Dumazet <edumazet@google.com>

[ 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.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/net/ip.h | 4 ++--
net/ipv4/route.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)

--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -352,7 +352,7 @@ 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(struct sock *sk,
@@ -364,7 +364,7 @@ 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);
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1268,7 +1268,7 @@ 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)