ipv6: properly prevent temp_prefered_lft sysctl race

The check for an underflow of tmp_prefered_lft is always false
because tmp_prefered_lft is unsigned. The intention of the check
was to guard against racing with an update of the
temp_prefered_lft sysctl, potentially resulting in an underflow.

As suggested by David Miller, the best way to prevent the race is
by reading the sysctl variable using READ_ONCE.

Signed-off-by: Jiri Bohac <[e-mail]>
Reported-by: Julia Lawall <[e-mail]>
Fixes: 76506a986dc3 ("IPv6: fix DESYNC_FACTOR")
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/addrconf.c

···
 	u32 addr_flags;
 	unsigned long now = jiffies;
 	long max_desync_factor;
+	s32 cnf_temp_preferred_lft;
 
 	write_lock_bh(&idev->lock);
 	if (ift) {
···
 	/* recalculate max_desync_factor each time and update
 	 * idev->desync_factor if it's larger
 	 */
+	cnf_temp_preferred_lft = READ_ONCE(idev->cnf.temp_prefered_lft);
 	max_desync_factor = min_t(__u32,
 				  idev->cnf.max_desync_factor,
-				  idev->cnf.temp_prefered_lft - regen_advance);
+				  cnf_temp_preferred_lft - regen_advance);
 
 	if (unlikely(idev->desync_factor > max_desync_factor)) {
 		if (max_desync_factor > 0) {
···
 	tmp_valid_lft = min_t(__u32,
 			      ifp->valid_lft,
 			      idev->cnf.temp_valid_lft + age);
-	tmp_prefered_lft = idev->cnf.temp_prefered_lft + age -
+	tmp_prefered_lft = cnf_temp_preferred_lft + age -
 			    idev->desync_factor;
-	/* guard against underflow in case of concurrent updates to cnf */
-	if (unlikely(tmp_prefered_lft < 0))
-		tmp_prefered_lft = 0;
 	tmp_prefered_lft = min_t(__u32, ifp->prefered_lft, tmp_prefered_lft);
 	tmp_plen = ifp->prefix_len;
 	tmp_tstamp = ifp->tstamp;