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

[Nikolay Borisov <nborisov@suse.com>]

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).

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

> +	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;
>  }
>