vfs_get_tree(): evict the call of security_sb_kern_mount()

Right now vfs_get_tree() calls security_sb_kern_mount() (i.e.
mount MAC) unless it gets MS_KERNMOUNT or MS_SUBMOUNT in flags.
Doing it that way is both clumsy and imprecise.

Consider the callers' tree of vfs_get_tree():
vfs_get_tree()
        <- do_new_mount()
	<- vfs_kern_mount()
		<- simple_pin_fs()
		<- vfs_submount()
		<- kern_mount_data()
		<- init_mount_tree()
		<- btrfs_mount()
			<- vfs_get_tree()
		<- nfs_do_root_mount()
			<- nfs4_try_mount()
				<- nfs_fs_mount()
					<- vfs_get_tree()
			<- nfs4_referral_mount()

do_new_mount() always does need MAC (we are guaranteed that neither
MS_KERNMOUNT nor MS_SUBMOUNT will be passed there).

simple_pin_fs(), vfs_submount() and kern_mount_data() pass explicit
flags inhibiting that check.  So does nfs4_referral_mount() (the
flags there are ulimately coming from vfs_submount()).

init_mount_tree() is called too early for anything LSM-related; it
doesn't matter whether we attempt those checks, they'll do nothing.

Finally, in case of btrfs_mount() and nfs_fs_mount(), doing MAC
is pointless - either the caller will do it, or the flags are
such that we wouldn't have done it either.

In other words, the one and only case when we want that check
done is when we are called from do_new_mount(), and there we
want it unconditionally.

So let's simply move it there.  The superblock is still locked,
so nobody is going to get access to it (via ustat(2), etc.)
until we get a chance to apply the checks - we are free to
move them to any point up to where we drop ->s_umount (in
do_new_mount_fc()).
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fs_context.c

@@ -90,6 +90,14 @@ struct fs_context *fs_context_for_mount(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(fs_context_for_mount);
 
+void fc_drop_locked(struct fs_context *fc)
+{
+	struct super_block *sb = fc->root->d_sb;
+	dput(fc->root);
+	fc->root = NULL;
+	deactivate_locked_super(sb);
+}
+
 static void legacy_fs_context_free(struct fs_context *fc);
 /**
  * put_fs_context - Dispose of a superblock configuration context.

fs/internal.h

@@ -57,6 +57,7 @@ extern void __init chrdev_init(void);
  */
 extern int legacy_get_tree(struct fs_context *fc);
 extern int parse_monolithic_mount_data(struct fs_context *, void *);
+extern void fc_drop_locked(struct fs_context *);
 
 /*
  * namei.c

fs/namespace.c

@@ -2536,11 +2536,13 @@ static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
 	struct super_block *sb = fc->root->d_sb;
 	int error;
 
-	if (mount_too_revealing(sb, &mnt_flags)) {
-		dput(fc->root);
-		fc->root = NULL;
-		deactivate_locked_super(sb);
-		return -EPERM;
+	error = security_sb_kern_mount(sb);
+	if (!error && mount_too_revealing(sb, &mnt_flags))
+		error = -EPERM;
+
+	if (unlikely(error)) {
+		fc_drop_locked(fc);
+		return error;
 	}
 
 	up_write(&sb->s_umount);

fs/super.c

@@ -1277,13 +1277,9 @@ int vfs_get_tree(struct fs_context *fc)
 	sb->s_flags |= SB_BORN;
 
 	error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
-	if (error)
-		goto out_sb;
-
-	if (!(fc->sb_flags & (MS_KERNMOUNT|MS_SUBMOUNT))) {
-		error = security_sb_kern_mount(sb);
-		if (error)
-			goto out_sb;
+	if (unlikely(error)) {
+		fc_drop_locked(fc);
+		return error;
 	}
 
 	/*
@@ -1296,11 +1292,6 @@ int vfs_get_tree(struct fs_context *fc)
 		"negative value (%lld)\n", fc->fs_type->name, sb->s_maxbytes);
 
 	return 0;
-out_sb:
-	dput(fc->root);
-	fc->root = NULL;
-	deactivate_locked_super(sb);
-	return error;
 }
 EXPORT_SYMBOL(vfs_get_tree);