[PATCH 4.19] VFS: Fix fuseblk memory leak caused by mount concurrency

[ChenXiaoSong]

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 adding a write lock while calling fs_set_subtype.

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: 79c0b2df79eb (add filesystem subtype support)
Cc: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: ChenXiaoSong <chenxiaosong2@xxxxxxxxxx>
---
 fs/namespace.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 2f3c6a0350a8..396ff1bcfdad 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2490,9 +2490,12 @@ 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);
+	if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE)) {
+		down_write(&mnt->mnt_sb->s_umount);
+		if (!mnt->mnt_sb->s_subtype)
+			mnt = fs_set_subtype(mnt, fstype);
+		up_write(&mnt->mnt_sb->s_umount);
+	}
 
 	put_filesystem(type);
 	if (IS_ERR(mnt))
-- 
2.25.4