Re: [PATCH v4 4/4] btrfs: ctree: Checking key orders before merged tree blocks

[David Sterba <dsterba@suse.cz>]

On Wed, Aug 12, 2020 at 02:05:09PM +0800, Qu Wenruo wrote:
> [BUG]
> With crafted image, btrfs can panic at btrfs_del_csums().
>   kernel BUG at fs/btrfs/ctree.c:3188!
>   invalid opcode: 0000 [#1] SMP PTI
>   CPU: 0 PID: 1156 Comm: btrfs-transacti Not tainted 5.0.0-rc8+ #9
>   RIP: 0010:btrfs_set_item_key_safe+0x16c/0x180
>   Code: b7 48 8d 7d bf 4c 89 fe 48 89 45 c8 0f b6 45 b6 88 45 c7 48 8b 45 ae 48 89 45 bf e8 ce f2 ff ff 85 c0 0f 8f 48 ff ff ff 0f 0b <0f> 0b e8 dd 8d be ff 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 66 66
>   RSP: 0018:ffff976141257ab8 EFLAGS: 00010202
>   RAX: 0000000000000001 RBX: ffff898a6b890930 RCX: 0000000004b70000
>   RDX: 0000000000000000 RSI: ffff976141257bae RDI: ffff976141257acf
>   RBP: ffff976141257b10 R08: 0000000000001000 R09: ffff9761412579a8
>   R10: 0000000000000000 R11: 0000000000000000 R12: ffff976141257abe
>   R13: 0000000000000003 R14: ffff898a6a8be578 R15: ffff976141257bae
>   FS: 0000000000000000(0000) GS:ffff898a77a00000(0000) knlGS:0000000000000000
>   CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 00007f779d9cd624 CR3: 000000022b2b4006 CR4: 00000000000206f0
>   Call Trace:
>   truncate_one_csum+0xac/0xf0
>   btrfs_del_csums+0x24f/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 93bf9db00e6c374e ]---
> 
> [CAUSE]
> This crafted image has a very tricky key order corruption:
> 
>   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 73785344) block 29757440 gen 19
>           key (EXTENT_CSUM EXTENT_CSUM 77594624) block 29753344 gen 19
>           ...
> 
>   leaf 29757440 items 5 free space 150 generation 19 owner CSUM_TREE
>           item 0 key (EXTENT_CSUM EXTENT_CSUM 73785344) itemoff 2323 itemsize 1672
>                   range start 73785344 end 75497472 length 1712128
>           item 1 key (EXTENT_CSUM EXTENT_CSUM 75497472) itemoff 2319 itemsize 4
>                   range start 75497472 end 75501568 length 4096
>           item 2 key (EXTENT_CSUM EXTENT_CSUM 75501568) itemoff 579 itemsize 1740
>                   range start 75501568 end 77283328 length 1781760
>           item 3 key (EXTENT_CSUM EXTENT_CSUM 77283328) itemoff 575 itemsize 4
>                   range start 77283328 end 77287424 length 4096
>           item 4 key (EXTENT_CSUM EXTENT_CSUM 4120596480) itemoff 275 itemsize 300 <<<
>                   range start 4120596480 end 4120903680 length 307200
>   leaf 29753344 items 3 free space 1936 generation 19 owner CSUM_TREE
>           item 0 key (18446744073457893366 EXTENT_CSUM 77594624) itemoff 2323 itemsize 1672
>                   range start 77594624 end 79306752 length 1712128
>           ...
> 
> Note the item 4 key of leaf 29757440, which is obviously too large, and
> even larger than the first key of the next leaf.
> 
> However it still follows the key order in that tree block, thus tree
> checker is unable to detect it at read time, since tree checker can only
> work inside a leaf, thus such complex corruption can't be rejected in
> advance.
> 
> [FIX]
> The next timing to detect such problem is at tree block merge time,
> which is in push_node_left(), balance_node_right(), push_leaf_left() and
> push_leaf_right().
> 
> Now we check if the key order of the right most key of the left node is
> larger than the left most key of the right node.
> 
> By this we don't need to call the full tree-check, while still keeps the
> key order correct as key order in each node is already checked by tree
> checker thus we only need to check the above two slots.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202833
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/ctree.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 70e49d8d4f6c..497abb397ea1 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -3159,6 +3159,52 @@ void btrfs_set_item_key_safe(struct btrfs_fs_info *fs_info,
>  		fixup_low_keys(path, &disk_key, 1);
>  }
>  
> +/*
> + * Check the cross tree block key ordering.
> + *
> + * Tree-checker only works inside one tree block, thus the following
> + * corruption can not be rejected by tree-checker:
> + * Leaf @left			| Leaf @right
> + * --------------------------------------------------------------
> + * | 1 | 2 | 3 | 4 | 5 | f6 |   | 7 | 8 |
> + *
> + * Key f6 in leaf @left itself is valid, but not valid when the next
> + * key in leaf @right is 7.
> + * This can only be checked at tree block merge time.
> + * And since tree checker has ensured all key order in each tree block
> + * is correct, we only need to bother the last key of @left and the first
> + * key of @right.
> + */
> +static bool valid_cross_tree_key_order(struct extent_buffer *left,
> +				       struct extent_buffer *right)

I think this naming is confusing, my first thought was that keys from
two trees were being checked, but this is for two leaves in the same
tree.

The arguments should be constified.

Elsewhere we use a check_<something> naming scheme with return value
true - problem, and 0/false - all ok. The 'valid' is the reverse and
also not following the scheme.

> +{
> +	struct btrfs_key left_last;
> +	struct btrfs_key right_first;
> +	int level = btrfs_header_level(left);
> +	int nr_left = btrfs_header_nritems(left);
> +	int nr_right = btrfs_header_nritems(right);
> +
> +	/* No key to check in one of the tree blocks */
> +	if (!nr_left || !nr_right)
> +		return true;
> +	if (level) {

You don't need a temporary variable for one-time use, btrfs_header_level
is understandable here.