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 <[email protected]>

---
 fs/proc/proc_net.c          |    6 ++++++
 include/linux/proc_fs.h     |    5 +----
 include/net/net_namespace.h |   12 ++++++++++++
 net/core/dev.c              |    6 +++++-
 net/core/dev_mcast.c        |    6 +++++-
 net/netlink/af_netlink.c    |    6 +++++-
 net/wireless/wext.c         |    6 +++++-
 7 files changed, 39 insertions(+), 8 deletions(-)

fs/proc/proc_net.c +6 -0

index 358930a..85cc8e8 100644
@@ -51,6 +51,12 @@ void proc_net_remove(struct net *net, const char *name)
 }
 EXPORT_SYMBOL_GPL(proc_net_remove);
 
+struct net *get_proc_net(const struct inode *inode)
+{
+	return maybe_get_net(PDE_NET(PDE(inode)));
+}
+EXPORT_SYMBOL_GPL(get_proc_net);
+
 static struct proc_dir_entry *proc_net_shadow;
 
 static struct dentry *proc_net_shadow_dentry(struct dentry *parent,

include/linux/proc_fs.h +1 -4

index 5964670..20741f6 100644
@@ -270,10 +270,7 @@ static inline struct net *PDE_NET(struct proc_dir_entry *pde)
 	return pde->parent->data;
 }
 
-static inline struct net *PROC_NET(const struct inode *inode)
-{
-	return PDE_NET(PDE(inode));
-}
+struct net *get_proc_net(const struct inode *inode);
 
 struct proc_maps_private {
 	struct pid *pid;

include/net/net_namespace.h +12 -0

index fac42db..dda03f3 100644
@@ -39,6 +39,18 @@ 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))

net/core/dev.c +5 -1

index a22a95d..f119dc0 100644
@@ -2446,7 +2446,11 @@ static int dev_seq_open(struct inode *inode, struct file *file)
 	res =  seq_open(file, &dev_seq_ops);
 	if (!res) {
 		seq = file->private_data;
-		seq->private = get_net(PROC_NET(inode));
+		seq->private = get_proc_net(inode);
+		if (!seq->private) {
+			seq_release(inode, file);
+			res = -ENXIO;
+		}
 	}
 	return res;
 }

net/core/dev_mcast.c +5 -1

index 1c4f619..896b0ca 100644
@@ -246,7 +246,11 @@ static int dev_mc_seq_open(struct inode *inode, struct file *file)
 	res = seq_open(file, &dev_mc_seq_ops);
 	if (!res) {
 		seq = file->private_data;
-		seq->private = get_net(PROC_NET(inode));
+		seq->private = get_proc_net(inode);
+		if (!seq->private) {
+			seq_release(inode, file);
+			res = -ENXIO;
+		}
 	}
 	return res;
 }

net/netlink/af_netlink.c +5 -1

index 3029f86..dc9f8c2 100644
@@ -1859,7 +1859,11 @@ static int netlink_seq_open(struct inode *inode, struct file *file)
 
 	seq = file->private_data;
 	seq->private = iter;
-	iter->net = get_net(PROC_NET(inode));
+	iter->net = get_proc_net(inode);
+	if (!iter->net) {
+		seq_release_private(inode, file);
+		return -ENXIO;
+	}
 	return 0;
 }
 

net/wireless/wext.c +5 -1

index e8b3409..85e5f9d 100644
@@ -678,7 +678,11 @@ static int wireless_seq_open(struct inode *inode, struct file *file)
 	res = seq_open(file, &wireless_seq_ops);
 	if (!res) {
 		seq = file->private_data;
-		seq->private = get_net(PROC_NET(inode));
+		seq->private = get_proc_net(inode);
+		if (!seq->private) {
+			seq_release(inode, file);
+			res = -ENXIO;
+		}
 	}
 	return res;
 }

Signature · 1 line

1.5.3.rc6.17.g1911

Looks much better!

Acked-by Paul E. McKenney <[email protected]>