[PATCH linux-4.19.y] VFS: Fix fuseblk memory leak caused by mount concurrency

[ChenXiaoSong <chenxiaosong2@xxxxxxxxxx>]

If two processes mount same superblock, memory leak occurs:

CPU0               |  CPU1
do_new_mount       |  do_new_mount
  fs_set_subtype   |    fs_set_subtype
    kstrdup        |
                   |      kstrdup
    memrory leak   |

Fix this by moving fs_set_subtype to mount_fs before up_write(&sb->s_umount).

Linus's tree already have refactoring patchset [1], one of them can fix this bug:
	c30da2e981a7 (fuse: convert to use the new mount API)

Since we did not merge the refactoring patchset in this branch, I create this patch.

[1] https://patchwork.kernel.org/project/linux-fsdevel/patch/20190903113640.7984-3-mszeredi@xxxxxxxxxx/

Fixes: 9d412a43c3b2 (vfs: split off vfsmount-related parts of vfs_kern_mount())
Cc: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: ChenXiaoSong <chenxiaosong2@xxxxxxxxxx>
---
 fs/namespace.c | 26 --------------------------
 fs/super.c     | 30 ++++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 26 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 2f3c6a0350a8..556fdd3b6a4e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2402,29 +2402,6 @@ static int do_move_mount(struct path *path, const char *old_name)
 	return err;
 }
 
-static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
-{
-	int err;
-	const char *subtype = strchr(fstype, '.');
-	if (subtype) {
-		subtype++;
-		err = -EINVAL;
-		if (!subtype[0])
-			goto err;
-	} else
-		subtype = "";
-
-	mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
-	err = -ENOMEM;
-	if (!mnt->mnt_sb->s_subtype)
-		goto err;
-	return mnt;
-
- err:
-	mntput(mnt);
-	return ERR_PTR(err);
-}
-
 /*
  * add a mount into a namespace's mount tree
  */
@@ -2490,9 +2467,6 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
 		return -ENODEV;
 
 	mnt = vfs_kern_mount(type, sb_flags, name, data);
-	if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
-	    !mnt->mnt_sb->s_subtype)
-		mnt = fs_set_subtype(mnt, fstype);
 
 	put_filesystem(type);
 	if (IS_ERR(mnt))
diff --git a/fs/super.c b/fs/super.c
index 9fb4553c46e6..b181878753bb 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1240,6 +1240,30 @@ struct dentry *mount_single(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(mount_single);
 
+static int fs_set_subtype(struct super_block *sb)
+{
+	int err;
+	const char *fstype = sb->s_type->name;
+	const char *subtype = strchr(fstype, '.');
+	if (subtype) {
+		subtype++;
+		err = -EINVAL;
+		if (!subtype[0])
+			goto err;
+	} else {
+		subtype = "";
+	}
+
+	sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
+	err = -ENOMEM;
+	if (!sb->s_subtype)
+		goto err;
+	return 0;
+
+err:
+	return err;
+}
+
 struct dentry *
 mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
 {
@@ -1289,6 +1313,12 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
 	WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
 		"negative value (%lld)\n", type->name, sb->s_maxbytes);
 
+	if ((sb->s_type->fs_flags & FS_HAS_SUBTYPE) && !sb->s_subtype) {
+		error = fs_set_subtype(sb);
+		if (error)
+			goto out_sb;
+	}
+
 	up_write(&sb->s_umount);
 	free_secdata(secdata);
 	return root;
-- 
2.25.4