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ipv4: fix dst race in sk_dst_get() 

When IP route cache had been removed in linux-3.6, we broke assumption
that dst entries were all freed after rcu grace period. DST_NOCACHE
dst were supposed to be freed from dst_release(). But it appears
we want to keep such dst around, either in UDP sockets or tunnels.

In sk_dst_get() we need to make sure dst refcount is not 0
before incrementing it, or else we might end up freeing a dst
twice.

DST_NOCACHE set on a dst does not mean this dst can not be attached
to a socket or a tunnel.

Then, before actual freeing, we need to observe a rcu grace period
to make sure all other cpus can catch the fact the dst is no longer
usable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dormando <dormando@rydia.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2014-06-24 10:05:11 -07:00 committed by David S. Miller
parent 99e72a0fed
commit f886497212
3 changed files with 18 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
include/net/sock.h
View File

@ -1730,8 +1730,8 @@ sk_dst_get(struct sock *sk)
rcu_read_lock();
dst = rcu_dereference(sk->sk_dst_cache);
if (dst)
dst_hold(dst);
if (dst && !atomic_inc_not_zero(&dst->__refcnt))
dst = NULL;
rcu_read_unlock();
return dst;
}

16
net/core/dst.c
View File

@ -269,6 +269,15 @@ again:
}
EXPORT_SYMBOL(dst_destroy);
static void dst_destroy_rcu(struct rcu_head *head)
{
struct dst_entry *dst = container_of(head, struct dst_entry, rcu_head);
dst = dst_destroy(dst);
if (dst)
__dst_free(dst);
}
void dst_release(struct dst_entry *dst)
{
if (dst) {
@ -276,11 +285,8 @@ void dst_release(struct dst_entry *dst)
newrefcnt = atomic_dec_return(&dst->__refcnt);
WARN_ON(newrefcnt < 0);
if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt) {
dst = dst_destroy(dst);
if (dst)
__dst_free(dst);
}
if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)
call_rcu(&dst->rcu_head, dst_destroy_rcu);
}
}
EXPORT_SYMBOL(dst_release);

14
net/ipv4/ip_tunnel.c
View File

@ -73,12 +73,7 @@ static void __tunnel_dst_set(struct ip_tunnel_dst *idst,
{
struct dst_entry *old_dst;
if (dst) {
if (dst->flags & DST_NOCACHE)
dst = NULL;
else
dst_clone(dst);
}
dst_clone(dst);
old_dst = xchg((__force struct dst_entry **)&idst->dst, dst);
dst_release(old_dst);
}
@ -108,13 +103,14 @@ static struct rtable *tunnel_rtable_get(struct ip_tunnel *t, u32 cookie)
rcu_read_lock();
dst = rcu_dereference(this_cpu_ptr(t->dst_cache)->dst);
if (dst && !atomic_inc_not_zero(&dst->__refcnt))
dst = NULL;
if (dst) {
if (dst->obsolete && dst->ops->check(dst, cookie) == NULL) {
rcu_read_unlock();
tunnel_dst_reset(t);
return NULL;
dst_release(dst);
dst = NULL;
}
dst_hold(dst);
}
rcu_read_unlock();
return (struct rtable *)dst;