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ipv6: properly prevent temp_prefered_lft sysctl race

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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: 76506a986d ("IPv6: fix DESYNC_FACTOR")
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jiri Bohac 2016-10-20 12:29:26 +02:00 committed by David S. Miller
parent 8be0328e52
commit 7aa8e63f0d
1 changed files with 4 additions and 5 deletions
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9
net/ipv6/addrconf.c
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@ -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;