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 <jbohac@suse.cz>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Fixes: 76506a986dc3 ("IPv6: fix DESYNC_FACTOR")
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index cc7c26d..060dd99 100644 (file)
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -1185,6 +1185,7 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, struct inet6_ifaddr *i
        u32 addr_flags;
        unsigned long now = jiffies;
        long max_desync_factor;
+       s32 cnf_temp_preferred_lft;
 
        write_lock_bh(&idev->lock);
        if (ift) {
@@ -1228,9 +1229,10 @@ retry:
        /* 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) {
@@ -1245,11 +1247,8 @@ retry:
        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;