ip: Fix data-races around sysctl_ip_fwd_use_pmtu.

stable inclusion
from stable-v4.19.254
commit eb15262128b793e4b1d1c4514d3e6d19c3959764
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ
CVE: NA

--------------------------------

[ Upstream commit 60c158dc ]

While reading sysctl_ip_fwd_use_pmtu, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its readers.

Fixes: f87c10a8 ("ipv4: introduce ip_dst_mtu_maybe_forward and protect forwarding path against pmtu spoofing")
Signed-off-by: NKuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

include/net/ip.h

@@ -401,7 +401,7 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 	struct net *net = dev_net(dst->dev);
 	unsigned int mtu;
 
-	if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
+	if (READ_ONCE(net->ipv4.sysctl_ip_fwd_use_pmtu) ||
 	    ip_mtu_locked(dst) ||
 	    !forwarding)
 		return dst_mtu(dst);

net/ipv4/route.c

@@ -1415,7 +1415,7 @@ u32 ip_mtu_from_fib_result(struct fib_result *res, __be32 daddr)
 	struct net_device *dev = nh->nh_dev;
 	u32 mtu = 0;
 
-	if (dev_net(dev)->ipv4.sysctl_ip_fwd_use_pmtu ||
+	if (READ_ONCE(dev_net(dev)->ipv4.sysctl_ip_fwd_use_pmtu) ||
 	    fi->fib_metrics->metrics[RTAX_LOCK - 1] & (1 << RTAX_MTU))
 		mtu = fi->fib_mtu;