net: do not delay dst_entries_add() in dst_release()

dst_entries_add() uses per-cpu data that might be freed at netns
dismantle from ip6_route_net_exit() calling dst_entries_destroy()

Before ip6_route_net_exit() can be called, we release all
the dsts associated with this netns, via calls to dst_release(),
which waits an rcu grace period before calling dst_destroy()

dst_entries_add() use in dst_destroy() is racy, because
dst_entries_destroy() could have been called already.

Decrementing the number of dsts must happen sooner.

Notes:

1) in CONFIG_XFRM case, dst_destroy() can call
   dst_release_immediate(child), this might also cause UAF
   if the child does not have DST_NOCOUNT set.
   IPSEC maintainers might take a look and see how to address this.

2) There is also discussion about removing this count of dst,
   which might happen in future kernels.

Fixes: f886497 ("ipv4: fix dst race in sk_dst_get()")
Closes: https://lore.kernel.org/lkml/CANn89iLCCGsP7SFn9HKpvnKu96Td4KD08xf7aGtiYgZnkjaL=w@mail.gmail.com/T/
Reported-by: Naresh Kamboju <[email protected]>
Tested-by: Linux Kernel Functional Testing <[email protected]>
Tested-by: Naresh Kamboju <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Xin Long <[email protected]>
Cc: Steffen Klassert <[email protected]>
Reviewed-by: Xin Long <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

net/core/dst.c

@@ -109,9 +109,6 @@ static void dst_destroy(struct dst_entry *dst)
 		child = xdst->child;
 	}
 #endif
-	if (!(dst->flags & DST_NOCOUNT))
-		dst_entries_add(dst->ops, -1);
-
 	if (dst->ops->destroy)
 		dst->ops->destroy(dst);
 	netdev_put(dst->dev, &dst->dev_tracker);
@@ -159,17 +156,27 @@ void dst_dev_put(struct dst_entry *dst)
 }
 EXPORT_SYMBOL(dst_dev_put);
 
+static void dst_count_dec(struct dst_entry *dst)
+{
+	if (!(dst->flags & DST_NOCOUNT))
+		dst_entries_add(dst->ops, -1);
+}
+
 void dst_release(struct dst_entry *dst)
 {
-	if (dst && rcuref_put(&dst->__rcuref))
+	if (dst && rcuref_put(&dst->__rcuref)) {
+		dst_count_dec(dst);
 		call_rcu_hurry(&dst->rcu_head, dst_destroy_rcu);
+	}
 }
 EXPORT_SYMBOL(dst_release);
 
 void dst_release_immediate(struct dst_entry *dst)
 {
-	if (dst && rcuref_put(&dst->__rcuref))
+	if (dst && rcuref_put(&dst->__rcuref)) {
+		dst_count_dec(dst);
 		dst_destroy(dst);
+	}
 }
 EXPORT_SYMBOL(dst_release_immediate);