ip6mr: Fix potential Spectre v1 vulnerability

vr.mifi is indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

net/ipv6/ip6mr.c:1845 ip6mr_ioctl() warn: potential spectre issue 'mrt->vif_table' [r] (local cap)
net/ipv6/ip6mr.c:1919 ip6mr_compat_ioctl() warn: potential spectre issue 'mrt->vif_table' [r] (local cap)

Fix this by sanitizing vr.mifi before using it to index mrt->vif_table'

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6mr.c

@@ -52,6 +52,8 @@
 #include <net/ip6_checksum.h>
 #include <linux/netconf.h>
 
+#include <linux/nospec.h>
+
 struct ip6mr_rule {
 	struct fib_rule		common;
 };
@@ -1841,6 +1843,7 @@ int ip6mr_ioctl(struct sock *sk, int cmd, void __user *arg)
 			return -EFAULT;
 		if (vr.mifi >= mrt->maxvif)
 			return -EINVAL;
+		vr.mifi = array_index_nospec(vr.mifi, mrt->maxvif);
 		read_lock(&mrt_lock);
 		vif = &mrt->vif_table[vr.mifi];
 		if (VIF_EXISTS(mrt, vr.mifi)) {
@@ -1915,6 +1918,7 @@ int ip6mr_compat_ioctl(struct sock *sk, unsigned int cmd, void __user *arg)
 			return -EFAULT;
 		if (vr.mifi >= mrt->maxvif)
 			return -EINVAL;
+		vr.mifi = array_index_nospec(vr.mifi, mrt->maxvif);
 		read_lock(&mrt_lock);
 		vif = &mrt->vif_table[vr.mifi];
 		if (VIF_EXISTS(mrt, vr.mifi)) {