[NET]: Fix race when opening a proc file while a network namespace is exiting.

The problem:  proc_net files remember which network namespace the are
against but do not remember hold a reference count (as that would pin
the network namespace).   So we currently have a small window where
the reference count on a network namespace may be incremented when opening
a /proc file when it has already gone to zero.

To fix this introduce maybe_get_net and get_proc_net.

maybe_get_net increments the network namespace reference count only if it is
greater then zero, ensuring we don't increment a reference count after it
has gone to zero.

get_proc_net handles all of the magic to go from a proc inode to the network
namespace instance and call maybe_get_net on it.

PROC_NET the old accessor is removed so that we don't get confused and use
the wrong helper function.

Then I fix up the callers to use get_proc_net and handle the case case
where get_proc_net returns NULL.  In that case I return -ENXIO because
effectively the network namespace has already gone away so the files
we are trying to access don't exist anymore.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 3081b6e..ac8f8304 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -46,6 +46,18 @@ static inline struct net *get_net(struct net *net)
 	return net;
 }
 
+static inline struct net *maybe_get_net(struct net *net)
+{
+	/* Used when we know struct net exists but we
+	 * aren't guaranteed a previous reference count
+	 * exists.  If the reference count is zero this
+	 * function fails and returns NULL.
+	 */
+	if (!atomic_inc_not_zero(&net->count))
+		net = NULL;
+	return net;
+}
+
 static inline void put_net(struct net *net)
 {
 	if (atomic_dec_and_test(&net->count))