ipv4: Fix potential Spectre v1 vulnerability

vr.vifi 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/ipv4/ipmr.c:1616 ipmr_ioctl() warn: potential spectre issue 'mrt->vif_table' [r] (local cap)
net/ipv4/ipmr.c:1690 ipmr_compat_ioctl() warn: potential spectre issue 'mrt->vif_table' [r] (local cap)

Fix this by sanitizing vr.vifi 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: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipmr.c

@@ -69,6 +69,8 @@
 #include <net/nexthop.h>
 #include <net/switchdev.h>
 
+#include <linux/nospec.h>
+
 struct ipmr_rule {
 	struct fib_rule		common;
 };
@@ -1612,6 +1614,7 @@ int ipmr_ioctl(struct sock *sk, int cmd, void __user *arg)
 			return -EFAULT;
 		if (vr.vifi >= mrt->maxvif)
 			return -EINVAL;
+		vr.vifi = array_index_nospec(vr.vifi, mrt->maxvif);
 		read_lock(&mrt_lock);
 		vif = &mrt->vif_table[vr.vifi];
 		if (VIF_EXISTS(mrt, vr.vifi)) {
@@ -1686,6 +1689,7 @@ int ipmr_compat_ioctl(struct sock *sk, unsigned int cmd, void __user *arg)
 			return -EFAULT;
 		if (vr.vifi >= mrt->maxvif)
 			return -EINVAL;
+		vr.vifi = array_index_nospec(vr.vifi, mrt->maxvif);
 		read_lock(&mrt_lock);
 		vif = &mrt->vif_table[vr.vifi];
 		if (VIF_EXISTS(mrt, vr.vifi)) {