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>

1 files changed, 4 insertions, 5 deletions

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index cc7c26d05f45..060dd9922018 100644
--- 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;