btrfs: Make btrfs handle security mount options internally to avoid losing security label.

[BUG]
Originally when mount btrfs with "-o subvol=" mount option, btrfs will
lose all security lable.
And if the btrfs fs is mounted somewhere else, due to the lost of
security lable, SELinux will refuse to mount since the same super block
is being mounted using different security lable.

[REPRODUCER]
With SELinux enabled:
 #mkfs -t btrfs /dev/sda5
 #mount -o context=system_u:object_r:nfs_t:s0 /dev/sda5 /mnt/btrfs
 #btrfs subvolume create /mnt/btrfs/subvol
 #mount -o subvol=subvol,context=system_u:object_r:nfs_t:s0 /dev/sda5
  /mnt/test

kernel message:
SELinux: mount invalid.  Same superblock, different security settings
for (dev sda5, type btrfs)

[REASON]
This happens because btrfs will call vfs_kern_mount() and then
mount_subtree() to handle subvolume name lookup.
First mount will cut off all the security lables and when it comes to
the second vfs_kern_mount(), it has no security label now.

[FIX]
This patch will makes btrfs behavior much more like nfs,
which has the type flag FS_BINARY_MOUNTDATA,
making btrfs handles the security label internally.
So security label will be set in the real mount time and won't lose
label when use with "subvol=" mount option.

Reported-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index f7555e2767de..b94c1c76cd59 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -34,6 +34,7 @@
 #include <linux/pagemap.h>
 #include <linux/btrfs.h>
 #include <linux/workqueue.h>
+#include <linux/security.h>
 #include "extent_io.h"
 #include "extent_map.h"
 #include "async-thread.h"
@@ -1725,6 +1726,9 @@ struct btrfs_fs_info {
 
 	spinlock_t unused_bgs_lock;
 	struct list_head unused_bgs;
+
+	/* For btrfs to record security options */
+	struct security_mnt_opts security_opts;
 };
 
 struct btrfs_subvolume_writers {
@@ -3591,6 +3595,7 @@ static inline void free_fs_info(struct btrfs_fs_info *fs_info)
 	kfree(fs_info->uuid_root);
 	kfree(fs_info->super_copy);
 	kfree(fs_info->super_for_commit);
+	security_free_mnt_opts(&fs_info->security_opts);
 	kfree(fs_info);
 }