fs-verity: move structs needed for file signing to UAPI header

Although it isn't used directly by the ioctls,
"struct fsverity_descriptor" is required by userspace programs that need
to compute fs-verity file digests in a standalone way.  Therefore
it's also needed to sign files in a standalone way.

Similarly, "struct fsverity_formatted_digest" (previously called
"struct fsverity_signed_digest" which was misleading) is also needed to
sign files if the built-in signature verification is being used.

Therefore, move these structs to the UAPI header.

While doing this, try to make it clear that the signature-related fields
in fsverity_descriptor aren't used in the file digest computation.

Acked-by: Luca Boccassi <luca.boccassi@microsoft.com>
Link: https://lore.kernel.org/r/20201113211918.71883-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/filesystems/fsverity.rst

@@ -334,17 +334,13 @@ root hash as well as other fields such as the file size::
             __u8 hash_algorithm;    /* Merkle tree hash algorithm */
             __u8 log_blocksize;     /* log2 of size of data and tree blocks */
             __u8 salt_size;         /* size of salt in bytes; 0 if none */
-            __le32 sig_size;        /* must be 0 */
+            __le32 __reserved_0x04; /* must be 0 */
             __le64 data_size;       /* size of file the Merkle tree is built over */
             __u8 root_hash[64];     /* Merkle tree root hash */
             __u8 salt[32];          /* salt prepended to each hashed block */
             __u8 __reserved[144];   /* must be 0's */
     };
 
-Note that the ``sig_size`` field must be set to 0 for the purpose of
-computing the file measurement, even if a signature was provided (or
-will be provided) to `FS_IOC_ENABLE_VERITY`_.
-
 Built-in signature verification
 ===============================
 

fs/verity/fsverity_private.h

@@ -77,49 +77,12 @@ struct fsverity_info {
 	const struct inode *inode;
 };
 
-/*
- * Merkle tree properties.  The fs-verity file digest is the hash of this
- * structure excluding the signature and with the sig_size field set to 0.
- */
-struct fsverity_descriptor {
-	__u8 version;		/* must be 1 */
-	__u8 hash_algorithm;	/* Merkle tree hash algorithm */
-	__u8 log_blocksize;	/* log2 of size of data and tree blocks */
-	__u8 salt_size;		/* size of salt in bytes; 0 if none */
-	__le32 sig_size;	/* size of signature in bytes; 0 if none */
-	__le64 data_size;	/* size of file the Merkle tree is built over */
-	__u8 root_hash[64];	/* Merkle tree root hash */
-	__u8 salt[32];		/* salt prepended to each hashed block */
-	__u8 __reserved[144];	/* must be 0's */
-	__u8 signature[];	/* optional PKCS#7 signature */
-};
-
 /* Arbitrary limit to bound the kmalloc() size.  Can be changed. */
 #define FS_VERITY_MAX_DESCRIPTOR_SIZE	16384
 
 #define FS_VERITY_MAX_SIGNATURE_SIZE	(FS_VERITY_MAX_DESCRIPTOR_SIZE - \
 					 sizeof(struct fsverity_descriptor))
 
-/*
- * Format in which fs-verity file digests are signed in built-in signatures.
- * This is the same as 'struct fsverity_digest', except here some magic bytes
- * are prepended to provide some context about what is being signed in case the
- * same key is used for non-fsverity purposes, and here the fields have fixed
- * endianness.
- *
- * This struct is specific to the built-in signature verification support, which
- * is optional.  fs-verity users may also verify signatures in userspace, in
- * which case userspace is responsible for deciding on what bytes are signed.
- * This struct may still be used, but it doesn't have to be.  For example,
- * userspace could instead use a string like "sha256:$digest_as_hex_string".
- */
-struct fsverity_formatted_digest {
-	char magic[8];			/* must be "FSVerity" */
-	__le16 digest_algorithm;
-	__le16 digest_size;
-	__u8 digest[];
-};
-
 /* hash_algs.c */
 
 extern struct fsverity_hash_alg fsverity_hash_algs[];

include/uapi/linux/fsverity.h

@@ -34,6 +34,55 @@ struct fsverity_digest {
 	__u8 digest[];
 };
 
+/*
+ * Struct containing a file's Merkle tree properties.  The fs-verity file digest
+ * is the hash of this struct.  A userspace program needs this struct only if it
+ * needs to compute fs-verity file digests itself, e.g. in order to sign files.
+ * It isn't needed just to enable fs-verity on a file.
+ *
+ * Note: when computing the file digest, 'sig_size' and 'signature' must be left
+ * zero and empty, respectively.  These fields are present only because some
+ * filesystems reuse this struct as part of their on-disk format.
+ */
+struct fsverity_descriptor {
+	__u8 version;		/* must be 1 */
+	__u8 hash_algorithm;	/* Merkle tree hash algorithm */
+	__u8 log_blocksize;	/* log2 of size of data and tree blocks */
+	__u8 salt_size;		/* size of salt in bytes; 0 if none */
+#ifdef __KERNEL__
+	__le32 sig_size;
+#else
+	__le32 __reserved_0x04;	/* must be 0 */
+#endif
+	__le64 data_size;	/* size of file the Merkle tree is built over */
+	__u8 root_hash[64];	/* Merkle tree root hash */
+	__u8 salt[32];		/* salt prepended to each hashed block */
+	__u8 __reserved[144];	/* must be 0's */
+#ifdef __KERNEL__
+	__u8 signature[];
+#endif
+};
+
+/*
+ * Format in which fs-verity file digests are signed in built-in signatures.
+ * This is the same as 'struct fsverity_digest', except here some magic bytes
+ * are prepended to provide some context about what is being signed in case the
+ * same key is used for non-fsverity purposes, and here the fields have fixed
+ * endianness.
+ *
+ * This struct is specific to the built-in signature verification support, which
+ * is optional.  fs-verity users may also verify signatures in userspace, in
+ * which case userspace is responsible for deciding on what bytes are signed.
+ * This struct may still be used, but it doesn't have to be.  For example,
+ * userspace could instead use a string like "sha256:$digest_as_hex_string".
+ */
+struct fsverity_formatted_digest {
+	char magic[8];			/* must be "FSVerity" */
+	__le16 digest_algorithm;
+	__le16 digest_size;
+	__u8 digest[];
+};
+
 #define FS_IOC_ENABLE_VERITY	_IOW('f', 133, struct fsverity_enable_arg)
 #define FS_IOC_MEASURE_VERITY	_IOWR('f', 134, struct fsverity_digest)