selinux: allow multiple opens of /sys/fs/selinux/policy

commit a02cd68 upstream.

Currently there can only be a single open of /sys/fs/selinux/policy at
any time. This allows any process to block any other process from
reading the kernel policy. The original motivation seems to have been
a mix of preventing an inconsistent view of the policy size and
preventing userspace from allocating kernel memory without bound, but
this is arguably equally bad. Eliminate the policy_opened flag and
shrink the critical section that the policy mutex is held. While we
are making changes here, drop a couple of extraneous BUG_ONs.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/selinux/20100726193414.19538.64028.stgit@paris.rdu.redhat.com/
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: stephensmalley

security/selinux/selinuxfs.c

@@ -76,7 +76,6 @@ struct selinux_fs_info {
 	int *bool_pending_values;
 	struct dentry *class_dir;
 	unsigned long last_class_ino;
-	bool policy_opened;
 	unsigned long last_ino;
 	struct super_block *sb;
 };
@@ -340,44 +339,31 @@ struct policy_load_memory {
 
 static int sel_open_policy(struct inode *inode, struct file *filp)
 {
-	struct selinux_fs_info *fsi = inode->i_sb->s_fs_info;
 	struct policy_load_memory *plm = NULL;
 	int rc;
 
-	BUG_ON(filp->private_data);
-
-	mutex_lock(&selinux_state.policy_mutex);
-
 	rc = avc_has_perm(current_sid(), SECINITSID_SECURITY,
 			  SECCLASS_SECURITY, SECURITY__READ_POLICY, NULL);
 	if (rc)
-		goto err;
-
-	rc = -EBUSY;
-	if (fsi->policy_opened)
-		goto err;
+		return rc;
 
-	rc = -ENOMEM;
 	plm = kzalloc_obj(*plm);
 	if (!plm)
-		goto err;
+		return -ENOMEM;
 
+	mutex_lock(&selinux_state.policy_mutex);
 	rc = security_read_policy(&plm->data, &plm->len);
 	if (rc)
 		goto err;
-
 	if ((size_t)i_size_read(inode) != plm->len) {
 		inode_lock(inode);
 		i_size_write(inode, plm->len);
 		inode_unlock(inode);
 	}
-
-	fsi->policy_opened = 1;
+	mutex_unlock(&selinux_state.policy_mutex);
 
 	filp->private_data = plm;
 
-	mutex_unlock(&selinux_state.policy_mutex);
-
 	return 0;
 err:
 	mutex_unlock(&selinux_state.policy_mutex);
@@ -390,13 +376,8 @@ static int sel_open_policy(struct inode *inode, struct file *filp)
 
 static int sel_release_policy(struct inode *inode, struct file *filp)
 {
-	struct selinux_fs_info *fsi = inode->i_sb->s_fs_info;
 	struct policy_load_memory *plm = filp->private_data;
 
-	BUG_ON(!plm);
-
-	fsi->policy_opened = 0;
-
 	vfree(plm->data);
 	kfree(plm);