From 7aa8e63f0d0f2e0ae353632bca1ce75a258696c6 Mon Sep 17 00:00:00 2001 From: Jiri Bohac Date: Thu, 20 Oct 2016 12:29:26 +0200 Subject: [PATCH] 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 Reported-by: Julia Lawall Fixes: 76506a986dc3 ("IPv6: fix DESYNC_FACTOR") Signed-off-by: David S. Miller --- net/ipv6/addrconf.c | 9 ++++----- 1 file 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; -- 2.20.1