[PATCH next] btrfs: fix deadlock in btrfs_read_chunk_tree

[Edward Adam Davis <eadavis@qq.com>]

Remove the lock uuid_mutex outside of sget_fc() to avoid the deadlock
reported by [1].

[1]
-> #1 (&type->s_umount_key#41/1){+.+.}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
       down_write_nested+0x9d/0x200 kernel/locking/rwsem.c:1693
       alloc_super+0x204/0x970 fs/super.c:345
       sget_fc+0x329/0xa40 fs/super.c:761
       btrfs_get_tree_super fs/btrfs/super.c:1867 [inline]
       btrfs_get_tree_subvol fs/btrfs/super.c:2059 [inline]
       btrfs_get_tree+0x4c6/0x12d0 fs/btrfs/super.c:2093
       vfs_get_tree+0x8f/0x2b0 fs/super.c:1804
       do_new_mount+0x24a/0xa40 fs/namespace.c:3902
       do_mount fs/namespace.c:4239 [inline]
       __do_sys_mount fs/namespace.c:4450 [inline]
       __se_sys_mount+0x317/0x410 fs/namespace.c:4427
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (uuid_mutex){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3168 [inline]
       check_prevs_add kernel/locking/lockdep.c:3287 [inline]
       validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3911
       __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5240
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:747
       btrfs_read_chunk_tree+0xef/0x2170 fs/btrfs/volumes.c:7462
       open_ctree+0x17f2/0x3a10 fs/btrfs/disk-io.c:3458
       btrfs_fill_super fs/btrfs/super.c:984 [inline]
       btrfs_get_tree_super fs/btrfs/super.c:1922 [inline]
       btrfs_get_tree_subvol fs/btrfs/super.c:2059 [inline]
       btrfs_get_tree+0xc6f/0x12d0 fs/btrfs/super.c:2093
       vfs_get_tree+0x8f/0x2b0 fs/super.c:1804
       do_new_mount+0x24a/0xa40 fs/namespace.c:3902
       do_mount fs/namespace.c:4239 [inline]
       __do_sys_mount fs/namespace.c:4450 [inline]
       __se_sys_mount+0x317/0x410 fs/namespace.c:4427
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&type->s_umount_key#41/1);
                               lock(uuid_mutex);
                               lock(&type->s_umount_key#41/1);
  lock(uuid_mutex);

 *** DEADLOCK ***

Fixes: 7aacdf6feed1 ("btrfs: delay btrfs_open_devices() until super block is created")
Reported-by: syzbot+fa90fcaa28f5cd4b1fc1@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fa90fcaa28f5cd4b1fc1
Tested-by: syzbot+fa90fcaa28f5cd4b1fc1@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/btrfs/super.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 237e60b53192..c2ce1eb53ad7 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1864,11 +1864,10 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 	fs_devices = device->fs_devices;
 	fs_info->fs_devices = fs_devices;
 
+	mutex_unlock(&uuid_mutex);
 	sb = sget_fc(fc, btrfs_fc_test_super, set_anon_super_fc);
-	if (IS_ERR(sb)) {
-		mutex_unlock(&uuid_mutex);
+	if (IS_ERR(sb))
 		return PTR_ERR(sb);
-	}
 
 	set_device_specific_options(fs_info);
 
@@ -1887,6 +1886,7 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 		 * But the fs_info->fs_devices is not opened, we should not let
 		 * btrfs_free_fs_context() to close them.
 		 */
+		mutex_lock(&uuid_mutex);
 		fs_info->fs_devices = NULL;
 		mutex_unlock(&uuid_mutex);
 
@@ -1906,6 +1906,7 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 		 */
 		ASSERT(fc->s_fs_info == NULL);
 
+		mutex_lock(&uuid_mutex);
 		ret = btrfs_open_devices(fs_devices, mode, sb);
 		mutex_unlock(&uuid_mutex);
 		if (ret < 0) {
-- 
2.43.0