fs-verity: support reading signature with ioctl

Add support for FS_VERITY_METADATA_TYPE_SIGNATURE to
FS_IOC_READ_VERITY_METADATA.  This allows a userspace server program to
retrieve the built-in signature (if present) of a verity 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.

The ability for userspace to read the built-in signatures is also useful
because it allows a system that is using the in-kernel signature
verification to migrate to userspace signature verification.

This has been tested using a new xfstest which calls this ioctl via a
new subcommand for the 'fsverity' program from fsverity-utils.

Link: https://lore.kernel.org/r/20210115181819.34732-7-ebiggers@kernel.orgReviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/filesystems/fsverity.rst

@@ -236,6 +236,7 @@ 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
+   #define FS_VERITY_METADATA_TYPE_SIGNATURE       3
 
    struct fsverity_read_metadata_arg {
            __u64 metadata_type;
@@ -256,6 +257,10 @@ This ioctl takes in a pointer to the following structure::
 - ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity
   descriptor.  See `fs-verity descriptor`_.
 
+- ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the signature which was
+  passed to FS_IOC_ENABLE_VERITY, if any.  See `Built-in signature
+  verification`_.
+
 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
@@ -279,7 +284,9 @@ FS_IOC_READ_VERITY_METADATA can fail with the following errors:
 - ``EINTR``: the ioctl was interrupted before any data was read
 - ``EINVAL``: reserved fields were set, or ``offset + length``
   overflowed
-- ``ENODATA``: the file is not a verity file
+- ``ENODATA``: the file is not a verity file, or
+  FS_VERITY_METADATA_TYPE_SIGNATURE was requested but the file doesn't
+  have a built-in signature
 - ``ENOTTY``: this type of filesystem does not implement fs-verity, or
   this ioctl is not yet implemented on it
 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity

fs/verity/read_metadata.c

@@ -114,6 +114,34 @@ static int fsverity_read_descriptor(struct inode *inode,
 	kfree(desc);
 	return res;
 }
+
+static int fsverity_read_signature(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;
+
+	if (desc->sig_size == 0) {
+		res = -ENODATA;
+		goto out;
+	}
+
+	/*
+	 * Include only the signature.  Note that fsverity_get_descriptor()
+	 * already verified that sig_size is in-bounds.
+	 */
+	res = fsverity_read_buffer(buf, offset, length, desc->signature,
+				   le32_to_cpu(desc->sig_size));
+out:
+	kfree(desc);
+	return res;
+}
+
 /**
  * fsverity_ioctl_read_metadata() - read verity metadata from a file
  * @filp: file to read the metadata from
@@ -158,6 +186,8 @@ int fsverity_ioctl_read_metadata(struct file *filp, const void __user *uarg)
 						 length);
 	case FS_VERITY_METADATA_TYPE_DESCRIPTOR:
 		return fsverity_read_descriptor(inode, buf, arg.offset, length);
+	case FS_VERITY_METADATA_TYPE_SIGNATURE:
+		return fsverity_read_signature(inode, buf, arg.offset, length);
 	default:
 		return -EINVAL;
 	}

include/uapi/linux/fsverity.h

@@ -85,6 +85,7 @@ struct fsverity_formatted_digest {
 
 #define FS_VERITY_METADATA_TYPE_MERKLE_TREE	1
 #define FS_VERITY_METADATA_TYPE_DESCRIPTOR	2
+#define FS_VERITY_METADATA_TYPE_SIGNATURE	3
 
 struct fsverity_read_metadata_arg {
 	__u64 metadata_type;