[PATCH 1/2] btrfs: reloc: fix reloc root leak and NULL pointer dereference

[PATCH 1/2] btrfs: reloc: fix reloc root leak and NULL pointer dereference

[Qu Wenruo]

commit 51415b6c1b117e223bc083e30af675cb5c5498f3 upstream.

[BUG]
When balance is canceled, there is a pretty high chance that unmounting
the fs can lead to lead the NULL pointer dereference:

  BTRFS warning (device dm-3): page private not zero on page 223158272
  ...
  BTRFS warning (device dm-3): page private not zero on page 223162368
  BTRFS error (device dm-3): leaked root 18446744073709551608-304 refcount 1
  BUG: kernel NULL pointer dereference, address: 0000000000000168
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 2 PID: 5793 Comm: umount Tainted: G           O      5.7.0-rc5-custom+ #53
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:__lock_acquire+0x5dc/0x24c0
  Call Trace:
   lock_acquire+0xab/0x390
   _raw_spin_lock+0x39/0x80
   btrfs_release_extent_buffer_pages+0xd7/0x200 [btrfs]
   release_extent_buffer+0xb2/0x170 [btrfs]
   free_extent_buffer+0x66/0xb0 [btrfs]
   btrfs_put_root+0x8e/0x130 [btrfs]
   btrfs_check_leaked_roots.cold+0x5/0x5d [btrfs]
   btrfs_free_fs_info+0xe5/0x120 [btrfs]
   btrfs_kill_super+0x1f/0x30 [btrfs]
   deactivate_locked_super+0x3b/0x80
   deactivate_super+0x3e/0x50
   cleanup_mnt+0x109/0x160
   __cleanup_mnt+0x12/0x20
   task_work_run+0x67/0xa0
   exit_to_usermode_loop+0xc5/0xd0
   syscall_return_slowpath+0x205/0x360
   do_syscall_64+0x6e/0xb0
   entry_SYSCALL_64_after_hwframe+0x49/0xb3
  RIP: 0033:0x7fd028ef740b

[CAUSE]
When balance is canceled, all reloc roots are marked as orphan, and
orphan reloc roots are going to be cleaned up.

However for orphan reloc roots and merged reloc roots, their lifespan
are quite different:

	Merged reloc roots	|	Orphan reloc roots by cancel
--------------------------------------------------------------------
create_reloc_root()		| create_reloc_root()
|- refs == 1			| |- refs == 1
				|
btrfs_grab_root(reloc_root);	| btrfs_grab_root(reloc_root);
|- refs == 2			| |- refs == 2
				|
root->reloc_root = reloc_root;	| root->reloc_root = reloc_root;
		>>> No difference so far <<<
				|
prepare_to_merge()		| prepare_to_merge()
|- btrfs_set_root_refs(item, 1);| |- if (!err) (err == -EINTR)
				|
merge_reloc_roots()		| merge_reloc_roots()
|- merge_reloc_root()		| |- Doing nothing to put reloc root
   |- insert_dirty_subvol()	| |- refs == 2
      |- __del_reloc_root()	|
         |- btrfs_put_root()	|
            |- refs == 1	|
		>>> Now orphan reloc roots still have refs 2 <<<
				|
clean_dirty_subvols()		| clean_dirty_subvols()
|- btrfs_drop_snapshot()	| |- btrfS_drop_snapshot()
   |- reloc_root get freed	|    |- reloc_root still has refs 2
				|	related ebs get freed, but
				|	reloc_root still recorded in
				|	allocated_roots
btrfs_check_leaked_roots()	| btrfs_check_leaked_roots()
|- No leaked roots		| |- Leaked reloc_roots detected
				| |- btrfs_put_root()
				|    |- free_extent_buffer(root->node);
				|       |- eb already freed, caused NULL
				|	   pointer dereference

[FIX]
The fix is to clear fs_root->reloc_root and put it at
merge_reloc_roots() time, so that we won't leak reloc roots.

Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
CC: stable@vger.kernel.org # 5.4.x
Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
[Manually solve the conflicts due to no btrfs root refs rework]
Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/relocation.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index e890f09e2073..3f8c706d75ee 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2525,12 +2525,10 @@ void merge_reloc_roots(struct reloc_control *rc)
 		reloc_root = list_entry(reloc_roots.next,
 					struct btrfs_root, root_list);
 
+		root = read_fs_root(fs_info, reloc_root->root_key.offset);
 		if (btrfs_root_refs(&reloc_root->root_item) > 0) {
-			root = read_fs_root(fs_info,
-					    reloc_root->root_key.offset);
 			BUG_ON(IS_ERR(root));
 			BUG_ON(root->reloc_root != reloc_root);
-
 			ret = merge_reloc_root(rc, root);
 			if (ret) {
 				if (list_empty(&reloc_root->root_list))
@@ -2539,6 +2537,11 @@ void merge_reloc_roots(struct reloc_control *rc)
 				goto out;
 			}
 		} else {
+			if (!IS_ERR(root)) {
+				if (root->reloc_root == reloc_root)
+					root->reloc_root = NULL;
+			}
+
 			list_del_init(&reloc_root->root_list);
 			/* Don't forget to queue this reloc root for cleanup */
 			list_add_tail(&reloc_root->reloc_dirty_list,
-- 
2.27.0

[PATCH 2/2] btrfs: reloc: clear DEAD_RELOC_TREE bit for orphan roots to prevent runaway balance

[Qu Wenruo]

commit 1dae7e0e58b484eaa43d530f211098fdeeb0f404 upstream.

[BUG]
There are several reported runaway balance, that balance is flooding the
log with "found X extents" where the X never changes.

[CAUSE]
Commit d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after
merge_reloc_roots") introduced BTRFS_ROOT_DEAD_RELOC_TREE bit to
indicate that one subvolume has finished its tree blocks swap with its
reloc tree.

However if balance is canceled or hits ENOSPC halfway, we didn't clear
the BTRFS_ROOT_DEAD_RELOC_TREE bit, leaving that bit hanging forever
until unmount.

Any subvolume root with that bit, would cause backref cache to skip this
tree block, as it has finished its tree block swap.  This would cause
all tree blocks of that root be ignored by balance, leading to runaway
balance.

[FIX]
Fix the problem by also clearing the BTRFS_ROOT_DEAD_RELOC_TREE bit for
the original subvolume of orphan reloc root.

Add an umount check for the stale bit still set.

Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
Cc: <stable@vger.kernel.org> # 5.4.x
Signed-off-by: Qu Wenruo <wqu@suse.com>
[Manually solve the conflicts due to no btrfs root refs rework]
Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/relocation.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 3f8c706d75ee..1b087ee338cc 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2540,6 +2540,8 @@ void merge_reloc_roots(struct reloc_control *rc)
 			if (!IS_ERR(root)) {
 				if (root->reloc_root == reloc_root)
 					root->reloc_root = NULL;
+				clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE,
+					  &root->state);
 			}
 
 			list_del_init(&reloc_root->root_list);
-- 
2.27.0