Re: [PATCH 1/2] btrfs: tree-checker: Add checker for variable length item

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2017年11月02日 20:06, Nikolay Borisov wrote:
> 
> 
> On  1.11.2017 14:14, Qu Wenruo wrote:
>> For the following types, we have items with variable length:
>> (With BTRFS_ prefix and _KEY suffix snipped)
>>
>> DIR_ITEM
>> DIR_INDEX
>> XATTR_ITEM
>> INODE_REF
>> INODE_EXTREF
>> ROOT_REF
>> ROOT_BACKREF
>>
>> They all use @name_len to indicate their name length, and XATTR_ITEM has
>> extra @data_len to indicate it data length.
>>
>> Despite their variable length, it's also possible to have several such
>> structure inside one item.
>>
>> This patch will add checker to ensure:
>>
>> 1) No structure header and its data cross item boundary
>> 2) Except XATTR_ITEM, no structure should have non-zero @data_len
>>
>> This checker is especially useful to avoid possible access beyond
>> boundary for fuzzed image.
>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/tree-checker.c | 123 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 123 insertions(+)
>>
>> diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
>> index 114fc5f0ecc5..f26e86fcbd74 100644
>> --- a/fs/btrfs/tree-checker.c
>> +++ b/fs/btrfs/tree-checker.c
>> @@ -222,6 +222,120 @@ static int check_csum_item(struct btrfs_root *root, struct extent_buffer *leaf,
>>  	return 0;
>>  }
>>  
>> +static u32 get_header_size(u8 type)
>> +{
>> +	switch (type) {
>> +	case BTRFS_DIR_ITEM_KEY:
>> +	case BTRFS_DIR_INDEX_KEY:
>> +	case BTRFS_XATTR_ITEM_KEY:
>> +		return sizeof(struct btrfs_dir_item);
>> +	case BTRFS_INODE_REF_KEY:
>> +		return sizeof(struct btrfs_inode_ref);
>> +	case BTRFS_INODE_EXTREF_KEY:
>> +		return sizeof(struct btrfs_inode_extref);
>> +	case BTRFS_ROOT_REF_KEY:
>> +	case BTRFS_ROOT_BACKREF_KEY:
>> +		return sizeof(struct btrfs_root_ref);
>> +	}
>> +	WARN_ON(1);
>> +	return 0;
>> +}
>> +
>> +static u16 get_header_namelen(struct extent_buffer *leaf, u8 type,
>> +			      u32 header_offset)
>> +{
>> +	/*
>> +	 * @header_offset is offset starts after leaf header, while the
>> +	 * accessors expects offset starts from leaf header.
>> +	 * Sowe need to adds LEAF_DATA_OFFSET here
>> +	 */
> 
> The comment is confusing (but so are the internals of accessing an
> extent buffer and all the offset magic happening behind the scenes).

The sentence inside the brackets is the point. :)

> 
> You are essentially opencoding btrfs_item_ptr_offset but instead of
> pointing at the first entry in the data portion, you really point to
> 'cur' entry and in order to make it "accessor friendly" you do the +
> BTRFS_LEAF_DATA_OFFSET. I'd say just put something like :
> 
> "This is the same as btrfs_item_ptr_offset but starting at item data
> pointed by header_offset". In fact I'd suggest renaming header_offset to
> data_offset

Only if we can ensure all offsets passed in are starting after header,
then I'm OK to rename it to "data_offset"

The "header_offset" here is to info any caller that one should pass
"normal" offset here, to avoid possible LEAF_DATA_OFFSET hassle.

Thanks,
Qu

> 
>> +	unsigned long leaf_offset = header_offset + BTRFS_LEAF_DATA_OFFSET;
>> +
>> +	switch (type) {
>> +	case BTRFS_DIR_ITEM_KEY:
>> +	case BTRFS_DIR_INDEX_KEY:
>> +	case BTRFS_XATTR_ITEM_KEY:
>> +		return btrfs_dir_name_len(leaf, (void *)leaf_offset);
>> +	case BTRFS_INODE_REF_KEY:
>> +		return btrfs_inode_ref_name_len(leaf, (void *)leaf_offset);
>> +	case BTRFS_INODE_EXTREF_KEY:
>> +		return btrfs_inode_extref_name_len(leaf, (void *)leaf_offset);
>> +	case BTRFS_ROOT_REF_KEY:
>> +	case BTRFS_ROOT_BACKREF_KEY:
>> +		return btrfs_root_ref_name_len(leaf, (void *)leaf_offset);
>> +	}
>> +	WARN_ON(1);
>> +	return 0;
>> +}
>> +
>> +static u16 get_header_datalen(struct extent_buffer *leaf, u8 type,
>> +			      unsigned long header_offset)
>> +{
>> +	/* Same as get_header_namelen */
>> +	unsigned long leaf_offset = header_offset + BTRFS_LEAF_DATA_OFFSET;
>> +
>> +	switch (type) {
>> +	case BTRFS_DIR_ITEM_KEY:
>> +	case BTRFS_DIR_INDEX_KEY:
>> +	case BTRFS_XATTR_ITEM_KEY:
>> +		return btrfs_dir_data_len(leaf, (void *)leaf_offset);
>> +	}
>> +	return 0;
>> +}
>> +
>> +/*
>> + * For items with variable length, normally with namelen and tailing data.
>> + * Like INODE_REF or XATTR
>> + */
>> +static int check_variable_length_item(struct btrfs_root *root,
>> +				      struct extent_buffer *leaf,
>> +				      struct btrfs_key *key, int slot)
>> +{
>> +	u8 type = key->type;
>> +	u32 item_start = btrfs_item_offset_nr(leaf, slot);
>> +	u32 item_end = btrfs_item_end_nr(leaf, slot);
>> +	u32 header_size = get_header_size(type);
>> +	u32 total_size;
>> +	u32 cur = item_start;
> 
> nit: Rename cur to cur_offset to make it clear this is an offset.
> 
>> +
>> +	while (cur < item_end) {
>> +		u32 namelen;
>> +		u32 datalen;
>> +
>> +		/* header itself should not cross item boundary */
>> +		if (cur + header_size > item_end) {
>> +			generic_err(root, leaf, slot,
>> +				"structure header crosses item boundary, have %u expect (%u, %u]",
>> +				cur + header_size, cur, item_end);
>> +			return -EUCLEAN;
>> +		}
>> +
>> +		namelen = get_header_namelen(leaf, type, cur);
>> +		datalen = get_header_datalen(leaf, type, cur);
>> +
>> +		/* Only XATTR can own data */
>> +		if (type != BTRFS_XATTR_ITEM_KEY && datalen) {
>> +			generic_err(root, leaf, slot,
>> +				"item has invalid data len, have %u expect 0",
>> +				datalen);
>> +			return -EUCLEAN;
>> +		}
>> +
>> +		total_size = header_size + namelen + datalen;
>> +
>> +		/* header and name/data should not cross item boundary */
>> +		if (cur + total_size > item_end) {
>> +			generic_err(root, leaf, slot,
>> +				"structure data crosses item boundary, have %u expect (%u, %u]",
>> +				cur + total_size, cur + header_size, item_end);
>> +			return -EUCLEAN;
>> +		}
>> +
>> +		cur += total_size;
>> +	}
>> +	return 0;
>> +}
>> +
>>  /*
>>   * Common point to switch the item-specific validation.
>>   */
>> @@ -238,6 +352,15 @@ static int check_leaf_item(struct btrfs_root *root,
>>  	case BTRFS_EXTENT_CSUM_KEY:
>>  		ret = check_csum_item(root, leaf, key, slot);
>>  		break;
>> +	case BTRFS_DIR_ITEM_KEY:
>> +	case BTRFS_XATTR_ITEM_KEY:
>> +	case BTRFS_DIR_INDEX_KEY:
>> +	case BTRFS_INODE_REF_KEY:
>> +	case BTRFS_INODE_EXTREF_KEY:
>> +	case BTRFS_ROOT_REF_KEY:
>> +	case BTRFS_ROOT_BACKREF_KEY:
>> +		ret = check_variable_length_item(root, leaf, key, slot);
>> +		break;
>>  	}
>>  	return ret;
>>  }
>>
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