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

[Nikolay Borisov <nborisov@suse.com>]

On 25.07.19 г. 9:12 ч., Qu Wenruo wrote:
> [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;
> +	}

Shouldn't this check be before !first_key, e.g. we always want to verify
that a particular node has at least 1 item ?

>  
>  	/*
>  	 * 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;
>  	}
>  
>