ipv6: prevent a possible race condition with lifetimes

valid_lft, prefered_lft and tstamp are always accessed under the lock
"lock" in other places. Reading these without taking the lock may result
in inconsistencies regarding the calculation of the valid and preferred
variables since decisions are taken on these fields for those variables.

Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Niels Dossche <niels.dossche@ugent.be>
Link: https://lore.kernel.org/r/20220223131954.6570-1-niels.dossche@ugent.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/addrconf.c

@@ -4998,6 +4998,7 @@ static int inet6_fill_ifaddr(struct sk_buff *skb, struct inet6_ifaddr *ifa,
  nla_put_s32(skb, IFA_TARGET_NETNSID, args->netnsid))
  goto error;
 
+ spin_lock_bh(&ifa->lock);
  if (!((ifa->flags&IFA_F_PERMANENT) &&
  (ifa->prefered_lft == INFINITY_LIFE_TIME))) {
  preferred = ifa->prefered_lft;
@@ -5019,6 +5020,7 @@ static int inet6_fill_ifaddr(struct sk_buff *skb, struct inet6_ifaddr *ifa,
  preferred = INFINITY_LIFE_TIME;
  valid = INFINITY_LIFE_TIME;
  }
+ spin_unlock_bh(&ifa->lock);
 
  if (!ipv6_addr_any(&ifa->peer_addr)) {
  if (nla_put_in6_addr(skb, IFA_LOCAL, &ifa->addr) < 0 ||