Commit 7af6355e authored by Omar Sandoval's avatar Omar Sandoval Committed by Luis Henriques

Btrfs: lock superblock before remounting for rw subvol

commit 773cd04e upstream.

Since commit 0723a047 ("btrfs: allow mounting btrfs subvolumes with
different ro/rw options"), when mounting a subvolume read/write when
another subvolume has previously been mounted read-only, we first do a
remount. However, this should be done with the superblock locked, as per
sync_filesystem():

	/*
	 * We need to be protected against the filesystem going from
	 * r/o to r/w or vice versa.
	 */
	WARN_ON(!rwsem_is_locked(&sb->s_umount));

This WARN_ON can easily be hit with:

mkfs.btrfs -f /dev/vdb
mount /dev/vdb /mnt
btrfs subvol create /mnt/vol1
btrfs subvol create /mnt/vol2
umount /mnt
mount -oro,subvol=/vol1 /dev/vdb /mnt
mount -orw,subvol=/vol2 /dev/vdb /mnt2

Fixes: 0723a047 ("btrfs: allow mounting btrfs subvolumes with different ro/rw options")
Reviewed-by: default avatarDavid Sterba <dsterba@suse.cz>
Signed-off-by: default avatarOmar Sandoval <osandov@osandov.com>
Signed-off-by: default avatarChris Mason <clm@fb.com>
Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
parent c0f611d6
......@@ -1203,7 +1203,9 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
return ERR_CAST(mnt);
}
down_write(&mnt->mnt_sb->s_umount);
r = btrfs_remount(mnt->mnt_sb, &flags, NULL);
up_write(&mnt->mnt_sb->s_umount);
if (r < 0) {
/* FIXME: release vfsmount mnt ??*/
kfree(newargs);
......
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