[PATCH v2 3/5] btrfs: Detect unbalanced tree with empty leaf before crashing btree operations

[Qu Wenruo <wqu@suse.com>]

[BUG]
With crafted image, btrfs will panic at btree operations:
  kernel BUG at fs/btrfs/ctree.c:3894!
  invalid opcode: 0000 [#1] SMP PTI
  CPU: 0 PID: 1138 Comm: btrfs-transacti Not tainted 5.0.0-rc8+ #9
  RIP: 0010:__push_leaf_left+0x6b6/0x6e0
  Code: 00 00 48 98 48 8d 04 80 48 8d 74 80 65 e8 42 5a 04 00 48 8b bd 78 ff ff ff 8b bf 90 d0 00 00 89 7d 98 83 ef 65 e9 06 ff ff ff <0f> 0b 0f 0b 48 8b 85 78 ff ff ff 8b 90 90 d0 00 00 e9 eb fe ff ff
  RSP: 0018:ffffc0bd4128b990 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffffa0a4ab8f0e38 RCX: 0000000000000000
  RDX: ffffa0a280000000 RSI: 0000000000000000 RDI: ffffa0a4b3814000
  RBP: ffffc0bd4128ba38 R08: 0000000000001000 R09: ffffc0bd4128b948
  R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000240
  R13: ffffa0a4b556fb60 R14: ffffa0a4ab8f0af0 R15: ffffa0a4ab8f0af0
  FS: 0000000000000000(0000) GS:ffffa0a4b7a00000(0000) knlGS:0000000000000000
  CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f2461c80020 CR3: 000000022b32a006 CR4: 00000000000206f0
  Call Trace:
  ? _cond_resched+0x1a/0x50
  push_leaf_left+0x179/0x190
  btrfs_del_items+0x316/0x470
  btrfs_del_csums+0x215/0x3a0
  __btrfs_free_extent.isra.72+0x5a7/0xbe0
  __btrfs_run_delayed_refs+0x539/0x1120
  btrfs_run_delayed_refs+0xdb/0x1b0
  btrfs_commit_transaction+0x52/0x950
  ? start_transaction+0x94/0x450
  transaction_kthread+0x163/0x190
  kthread+0x105/0x140
  ? btrfs_cleanup_transaction+0x560/0x560
  ? kthread_destroy_worker+0x50/0x50
  ret_from_fork+0x35/0x40
  Modules linked in:
  ---[ end trace c2425e6e89b5558f ]---

[CAUSE]
The offending csum tree looks like this:
checksum tree key (CSUM_TREE ROOT_ITEM 0)
node 29741056 level 1 items 14 free 107 generation 19 owner CSUM_TREE
        ...
        key (EXTENT_CSUM EXTENT_CSUM 85975040) block 29630464 gen 17
        key (EXTENT_CSUM EXTENT_CSUM 89911296) block 29642752 gen 17 <<<
        key (EXTENT_CSUM EXTENT_CSUM 92274688) block 29646848 gen 17
        ...

leaf 29630464 items 6 free space 1 generation 17 owner CSUM_TREE
        item 0 key (EXTENT_CSUM EXTENT_CSUM 85975040) itemoff 3987 itemsize 8
                range start 85975040 end 85983232 length 8192
        ...
leaf 29642752 items 0 free space 3995 generation 17 owner 0
                    ^ empty leaf            invalid owner ^

leaf 29646848 items 1 free space 602 generation 17 owner CSUM_TREE
        item 0 key (EXTENT_CSUM EXTENT_CSUM 92274688) itemoff 627 itemsize 3368
                range start 92274688 end 95723520 length 3448832

So we have a corrupted csum tree where one tree leaf is completely
empty, causing unbalanced btree, thus leading to unexpected btree
balance error.

[FIX]
For this particular case, we handle it in two directions to catch it:
- Check if the tree block is empty through btrfs_verify_level_key()
  So that invalid tree blocks won't be read out through
  btrfs_search_slot() and its variants.

- Check 0 tree owner in tree checker
  NO tree is using 0 as its tree owner, detect it and reject at tree
  block read time.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=202821
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/disk-io.c      | 8 ++++++++
 fs/btrfs/tree-checker.c | 6 ++++++
 2 files changed, 14 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index deb74a8c191a..a843c21f3060 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -414,6 +414,14 @@ int btrfs_verify_level_key(struct extent_buffer *eb, int level,
 
 	if (!first_key)
 		return 0;
+	/* We have @first_key, so this @eb must have at least one item */
+	if (btrfs_header_nritems(eb) == 0) {
+		btrfs_err(fs_info,
+		"invalid tree nritems, bytenr=%llu nritems=0 expect >0",
+			  eb->start);
+		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
+		return -EUCLEAN;
+	}
 
 	/*
 	 * For live tree block (new tree blocks in current transaction),
diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
index 96fce4bef4e7..a4c7f7ed8490 100644
--- a/fs/btrfs/tree-checker.c
+++ b/fs/btrfs/tree-checker.c
@@ -888,6 +888,12 @@ static int check_leaf(struct extent_buffer *leaf, bool check_item_data)
 				    owner);
 			return -EUCLEAN;
 		}
+		/* Unknown tree */
+		if (owner == 0) {
+			generic_err(leaf, 0,
+				"invalid owner, root 0 is not defined");
+			return -EUCLEAN;
+		}
 		return 0;
 	}
 
-- 
2.22.0