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Date:   Wed, 27 Jan 2021 17:11:14 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fscrypt@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-api@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Victor Hsieh <victorhsieh@...gle.com>
Subject: Re: [PATCH 5/6] fs-verity: support reading descriptor with ioctl

On 01/15, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Add support for FS_VERITY_METADATA_TYPE_DESCRIPTOR to
> FS_IOC_READ_VERITY_METADATA.  This allows a userspace server program to
> retrieve the fs-verity descriptor of a file for serving to a client
> which implements fs-verity compatible verification.  See the patch which
> introduced FS_IOC_READ_VERITY_METADATA for more details.
> 
> "fs-verity descriptor" here means only the part that userspace cares
> about because it is hashed to produce the file digest.  It doesn't
> include the signature which ext4 and f2fs append to the
> fsverity_descriptor struct when storing it on-disk, since that way of
> storing the signature is an implementation detail.  The next patch adds
> a separate metadata_type value for retrieving the signature separately.
> 
> This has been tested using a new xfstest which calls this ioctl via a
> new subcommand for the 'fsverity' program from fsverity-utils.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Reviewed-by: Jaegeuk Kim <jaegeuk@...nel.org>

> ---
>  Documentation/filesystems/fsverity.rst |  4 +++
>  fs/verity/read_metadata.c              | 40 ++++++++++++++++++++++++++
>  include/uapi/linux/fsverity.h          |  1 +
>  3 files changed, 45 insertions(+)
> 
> diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst
> index 50b47a6d9ea11..6dc5772037ef9 100644
> --- a/Documentation/filesystems/fsverity.rst
> +++ b/Documentation/filesystems/fsverity.rst
> @@ -235,6 +235,7 @@ need this ioctl.
>  This ioctl takes in a pointer to the following structure::
>  
>     #define FS_VERITY_METADATA_TYPE_MERKLE_TREE     1
> +   #define FS_VERITY_METADATA_TYPE_DESCRIPTOR      2
>  
>     struct fsverity_read_metadata_arg {
>             __u64 metadata_type;
> @@ -252,6 +253,9 @@ This ioctl takes in a pointer to the following structure::
>    the same order that their hashes are themselves hashed.
>    See `Merkle tree`_ for more information.
>  
> +- ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity
> +  descriptor.  See `fs-verity descriptor`_.
> +
>  The semantics are similar to those of ``pread()``.  ``offset``
>  specifies the offset in bytes into the metadata item to read from, and
>  ``length`` specifies the maximum number of bytes to read from the
> diff --git a/fs/verity/read_metadata.c b/fs/verity/read_metadata.c
> index 0f8ad2991cf90..2dea6dd3bb05a 100644
> --- a/fs/verity/read_metadata.c
> +++ b/fs/verity/read_metadata.c
> @@ -76,6 +76,44 @@ static int fsverity_read_merkle_tree(struct inode *inode,
>  	}
>  	return retval ? retval : err;
>  }
> +
> +/* Copy the requested portion of the buffer to userspace. */
> +static int fsverity_read_buffer(void __user *dst, u64 offset, int length,
> +				const void *src, size_t src_length)
> +{
> +	if (offset >= src_length)
> +		return 0;
> +	src += offset;
> +	src_length -= offset;
> +
> +	length = min_t(size_t, length, src_length);
> +
> +	if (copy_to_user(dst, src, length))
> +		return -EFAULT;
> +
> +	return length;
> +}
> +
> +static int fsverity_read_descriptor(struct inode *inode,
> +				    void __user *buf, u64 offset, int length)
> +{
> +	struct fsverity_descriptor *desc;
> +	size_t desc_size;
> +	int res;
> +
> +	res = fsverity_get_descriptor(inode, &desc, &desc_size);
> +	if (res)
> +		return res;
> +
> +	/* don't include the signature */
> +	desc_size = offsetof(struct fsverity_descriptor, signature);
> +	desc->sig_size = 0;
> +
> +	res = fsverity_read_buffer(buf, offset, length, desc, desc_size);
> +
> +	kfree(desc);
> +	return res;
> +}
>  /**
>   * fsverity_ioctl_read_metadata() - read verity metadata from a file
>   * @filp: file to read the metadata from
> @@ -118,6 +156,8 @@ int fsverity_ioctl_read_metadata(struct file *filp, const void __user *uarg)
>  	case FS_VERITY_METADATA_TYPE_MERKLE_TREE:
>  		return fsverity_read_merkle_tree(inode, vi, buf, arg.offset,
>  						 length);
> +	case FS_VERITY_METADATA_TYPE_DESCRIPTOR:
> +		return fsverity_read_descriptor(inode, buf, arg.offset, length);
>  	default:
>  		return -EINVAL;
>  	}
> diff --git a/include/uapi/linux/fsverity.h b/include/uapi/linux/fsverity.h
> index 94003b153cb3d..41abc283dbccb 100644
> --- a/include/uapi/linux/fsverity.h
> +++ b/include/uapi/linux/fsverity.h
> @@ -84,6 +84,7 @@ struct fsverity_formatted_digest {
>  };
>  
>  #define FS_VERITY_METADATA_TYPE_MERKLE_TREE	1
> +#define FS_VERITY_METADATA_TYPE_DESCRIPTOR	2
>  
>  struct fsverity_read_metadata_arg {
>  	__u64 metadata_type;
> -- 
> 2.30.0

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