Merge tag 'fsverity-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt

Pull fsverity updates from Eric Biggers:
 "Add an ioctl which allows reading fs-verity metadata from a file.

  This is useful when a file with fs-verity enabled needs to be served
  somewhere, and the other end wants to do its own fs-verity compatible
  verification of the file. See the commit messages for details.

  This new ioctl has been tested using new xfstests I've written for it"

* tag 'fsverity-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt:
  fs-verity: support reading signature with ioctl
  fs-verity: support reading descriptor with ioctl
  fs-verity: support reading Merkle tree with ioctl
  fs-verity: add FS_IOC_READ_VERITY_METADATA ioctl
  fs-verity: don't pass whole descriptor to fsverity_verify_signature()
  fs-verity: factor out fsverity_get_descriptor()

Documentation/filesystems/fsverity.rst

@@ -217,6 +217,82 @@ FS_IOC_MEASURE_VERITY can fail with the following errors:
 - ``EOVERFLOW``: the digest is longer than the specified
   ``digest_size`` bytes.  Try providing a larger buffer.
 
+FS_IOC_READ_VERITY_METADATA
+---------------------------
+
+The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a
+verity file.  This ioctl is available since Linux v5.12.
+
+This ioctl allows writing a server program that takes a verity file
+and serves it to a client program, such that the client can do its own
+fs-verity compatible verification of the file.  This only makes sense
+if the client doesn't trust the server and if the server needs to
+provide the storage for the client.
+
+This is a fairly specialized use case, and most fs-verity users won't
+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
+   #define FS_VERITY_METADATA_TYPE_SIGNATURE       3
+
+   struct fsverity_read_metadata_arg {
+           __u64 metadata_type;
+           __u64 offset;
+           __u64 length;
+           __u64 buf_ptr;
+           __u64 __reserved;
+   };
+
+``metadata_type`` specifies the type of metadata to read:
+
+- ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the
+  Merkle tree.  The blocks are returned in order from the root level
+  to the leaf level.  Within each level, the blocks are returned in
+  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`_.
+
+- ``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
+metadata item.  ``buf_ptr`` is the pointer to the buffer to read into,
+cast to a 64-bit integer.  ``__reserved`` must be 0.  On success, the
+number of bytes read is returned.  0 is returned at the end of the
+metadata item.  The returned length may be less than ``length``, for
+example if the ioctl is interrupted.
+
+The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed
+to be authenticated against the file digest that would be returned by
+`FS_IOC_MEASURE_VERITY`_, as the metadata is expected to be used to
+implement fs-verity compatible verification anyway (though absent a
+malicious disk, the metadata will indeed match).  E.g. to implement
+this ioctl, the filesystem is allowed to just read the Merkle tree
+blocks from disk without actually verifying the path to the root node.
+
+FS_IOC_READ_VERITY_METADATA can fail with the following errors:
+
+- ``EFAULT``: the caller provided inaccessible memory
+- ``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, 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
+  support, or the filesystem superblock has not had the 'verity'
+  feature enabled on it.  (See `Filesystem support`_.)
+
 FS_IOC_GETFLAGS
 ---------------
 

fs/ext4/ioctl.c

@@ -1309,6 +1309,12 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 			return -EOPNOTSUPP;
 		return fsverity_ioctl_measure(filp, (void __user *)arg);
 
+	case FS_IOC_READ_VERITY_METADATA:
+		if (!ext4_has_feature_verity(sb))
+			return -EOPNOTSUPP;
+		return fsverity_ioctl_read_metadata(filp,
+						    (const void __user *)arg);
+
 	default:
 		return -ENOTTY;
 	}
@@ -1391,6 +1397,7 @@ long ext4_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case FS_IOC_GETFSMAP:
 	case FS_IOC_ENABLE_VERITY:
 	case FS_IOC_MEASURE_VERITY:
+	case FS_IOC_READ_VERITY_METADATA:
 	case EXT4_IOC_CLEAR_ES_CACHE:
 	case EXT4_IOC_GETSTATE:
 	case EXT4_IOC_GET_ES_CACHE:

fs/f2fs/file.c

@@ -3374,6 +3374,14 @@ static int f2fs_ioc_measure_verity(struct file *filp, unsigned long arg)
 	return fsverity_ioctl_measure(filp, (void __user *)arg);
 }
 
+static int f2fs_ioc_read_verity_metadata(struct file *filp, unsigned long arg)
+{
+	if (!f2fs_sb_has_verity(F2FS_I_SB(file_inode(filp))))
+		return -EOPNOTSUPP;
+
+	return fsverity_ioctl_read_metadata(filp, (const void __user *)arg);
+}
+
 static int f2fs_ioc_getfslabel(struct file *filp, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
@@ -4291,6 +4299,8 @@ static long __f2fs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		return f2fs_ioc_enable_verity(filp, arg);
 	case FS_IOC_MEASURE_VERITY:
 		return f2fs_ioc_measure_verity(filp, arg);
+	case FS_IOC_READ_VERITY_METADATA:
+		return f2fs_ioc_read_verity_metadata(filp, arg);
 	case FS_IOC_GETFSLABEL:
 		return f2fs_ioc_getfslabel(filp, arg);
 	case FS_IOC_SETFSLABEL:
@@ -4548,6 +4558,7 @@ long f2fs_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case F2FS_IOC_RESIZE_FS:
 	case FS_IOC_ENABLE_VERITY:
 	case FS_IOC_MEASURE_VERITY:
+	case FS_IOC_READ_VERITY_METADATA:
 	case FS_IOC_GETFSLABEL:
 	case FS_IOC_SETFSLABEL:
 	case F2FS_IOC_GET_COMPRESS_BLOCKS:

fs/verity/Makefile

@@ -5,6 +5,7 @@ obj-$(CONFIG_FS_VERITY) += enable.o \
 			   init.o \
 			   measure.o \
 			   open.o \
+			   read_metadata.o \
 			   verify.o
 
 obj-$(CONFIG_FS_VERITY_BUILTIN_SIGNATURES) += signature.o

fs/verity/fsverity_private.h

@@ -122,12 +122,17 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 				     const u8 *salt, size_t salt_size);
 
 struct fsverity_info *fsverity_create_info(const struct inode *inode,
-					   void *desc, size_t desc_size);
+					   struct fsverity_descriptor *desc,
+					   size_t desc_size);
 
 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi);
 
 void fsverity_free_info(struct fsverity_info *vi);
 
+int fsverity_get_descriptor(struct inode *inode,
+			    struct fsverity_descriptor **desc_ret,
+			    size_t *desc_size_ret);
+
 int __init fsverity_init_info_cache(void);
 void __init fsverity_exit_info_cache(void);
 
@@ -135,15 +140,13 @@ void __init fsverity_exit_info_cache(void);
 
 #ifdef CONFIG_FS_VERITY_BUILTIN_SIGNATURES
 int fsverity_verify_signature(const struct fsverity_info *vi,
-			      const struct fsverity_descriptor *desc,
-			      size_t desc_size);
+			      const u8 *signature, size_t sig_size);
 
 int __init fsverity_init_signature(void);
 #else /* !CONFIG_FS_VERITY_BUILTIN_SIGNATURES */
 static inline int
 fsverity_verify_signature(const struct fsverity_info *vi,
-			  const struct fsverity_descriptor *desc,
-			  size_t desc_size)
+			  const u8 *signature, size_t sig_size)
 {
 	return 0;
 }

fs/verity/open.c

@@ -142,45 +142,17 @@ static int compute_file_digest(struct fsverity_hash_alg *hash_alg,
 }
 
 /*
- * Validate the given fsverity_descriptor and create a new fsverity_info from
- * it.  The signature (if present) is also checked.
+ * Create a new fsverity_info from the given fsverity_descriptor (with optional
+ * appended signature), and check the signature if present.  The
+ * fsverity_descriptor must have already undergone basic validation.
  */
 struct fsverity_info *fsverity_create_info(const struct inode *inode,
-					   void *_desc, size_t desc_size)
+					   struct fsverity_descriptor *desc,
+					   size_t desc_size)
 {
-	struct fsverity_descriptor *desc = _desc;
 	struct fsverity_info *vi;
 	int err;
 
-	if (desc_size < sizeof(*desc)) {
-		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
-			     desc_size);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (desc->version != 1) {
-		fsverity_err(inode, "Unrecognized descriptor version: %u",
-			     desc->version);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
-		fsverity_err(inode, "Reserved bits set in descriptor");
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (desc->salt_size > sizeof(desc->salt)) {
-		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
-		return ERR_PTR(-EINVAL);
-	}
-
-	if (le64_to_cpu(desc->data_size) != inode->i_size) {
-		fsverity_err(inode,
-			     "Wrong data_size: %llu (desc) != %lld (inode)",
-			     le64_to_cpu(desc->data_size), inode->i_size);
-		return ERR_PTR(-EINVAL);
-	}
-
 	vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
 	if (!vi)
 		return ERR_PTR(-ENOMEM);
@@ -209,7 +181,8 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
 		 vi->tree_params.hash_alg->name,
 		 vi->tree_params.digest_size, vi->file_digest);
 
-	err = fsverity_verify_signature(vi, desc, desc_size);
+	err = fsverity_verify_signature(vi, desc->signature,
+					le32_to_cpu(desc->sig_size));
 out:
 	if (err) {
 		fsverity_free_info(vi);
@@ -245,15 +218,57 @@ void fsverity_free_info(struct fsverity_info *vi)
 	kmem_cache_free(fsverity_info_cachep, vi);
 }
 
-/* Ensure the inode has an ->i_verity_info */
-static int ensure_verity_info(struct inode *inode)
+static bool validate_fsverity_descriptor(struct inode *inode,
+					 const struct fsverity_descriptor *desc,
+					 size_t desc_size)
 {
-	struct fsverity_info *vi = fsverity_get_info(inode);
-	struct fsverity_descriptor *desc;
-	int res;
+	if (desc_size < sizeof(*desc)) {
+		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
+			     desc_size);
+		return false;
+	}
 
-	if (vi)
-		return 0;
+	if (desc->version != 1) {
+		fsverity_err(inode, "Unrecognized descriptor version: %u",
+			     desc->version);
+		return false;
+	}
+
+	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
+		fsverity_err(inode, "Reserved bits set in descriptor");
+		return false;
+	}
+
+	if (desc->salt_size > sizeof(desc->salt)) {
+		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
+		return false;
+	}
+
+	if (le64_to_cpu(desc->data_size) != inode->i_size) {
+		fsverity_err(inode,
+			     "Wrong data_size: %llu (desc) != %lld (inode)",
+			     le64_to_cpu(desc->data_size), inode->i_size);
+		return false;
+	}
+
+	if (le32_to_cpu(desc->sig_size) > desc_size - sizeof(*desc)) {
+		fsverity_err(inode, "Signature overflows verity descriptor");
+		return false;
+	}
+
+	return true;
+}
+
+/*
+ * Read the inode's fsverity_descriptor (with optional appended signature) from
+ * the filesystem, and do basic validation of it.
+ */
+int fsverity_get_descriptor(struct inode *inode,
+			    struct fsverity_descriptor **desc_ret,
+			    size_t *desc_size_ret)
+{
+	int res;
+	struct fsverity_descriptor *desc;
 
 	res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
 	if (res < 0) {
@@ -272,20 +287,46 @@ static int ensure_verity_info(struct inode *inode)
 	res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
 	if (res < 0) {
 		fsverity_err(inode, "Error %d reading verity descriptor", res);
-		goto out_free_desc;
+		kfree(desc);
+		return res;
+	}
+
+	if (!validate_fsverity_descriptor(inode, desc, res)) {
+		kfree(desc);
+		return -EINVAL;
 	}
 
-	vi = fsverity_create_info(inode, desc, res);
+	*desc_ret = desc;
+	*desc_size_ret = res;
+	return 0;
+}
+
+/* Ensure the inode has an ->i_verity_info */
+static int ensure_verity_info(struct inode *inode)
+{
+	struct fsverity_info *vi = fsverity_get_info(inode);
+	struct fsverity_descriptor *desc;
+	size_t desc_size;
+	int err;
+
+	if (vi)
+		return 0;
+
+	err = fsverity_get_descriptor(inode, &desc, &desc_size);
+	if (err)
+		return err;
+
+	vi = fsverity_create_info(inode, desc, desc_size);
 	if (IS_ERR(vi)) {
-		res = PTR_ERR(vi);
+		err = PTR_ERR(vi);
 		goto out_free_desc;
 	}
 
 	fsverity_set_info(inode, vi);
-	res = 0;
+	err = 0;
 out_free_desc:
 	kfree(desc);
-	return res;
+	return err;
 }
 
 /**

fs/verity/read_metadata.c

+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Ioctl to read verity metadata
+ *
+ * Copyright 2021 Google LLC
+ */
+
+#include "fsverity_private.h"
+
+#include <linux/backing-dev.h>
+#include <linux/highmem.h>
+#include <linux/sched/signal.h>
+#include <linux/uaccess.h>
+
+static int fsverity_read_merkle_tree(struct inode *inode,
+				     const struct fsverity_info *vi,
+				     void __user *buf, u64 offset, int length)
+{
+	const struct fsverity_operations *vops = inode->i_sb->s_vop;
+	u64 end_offset;
+	unsigned int offs_in_page;
+	pgoff_t index, last_index;
+	int retval = 0;
+	int err = 0;
+
+	end_offset = min(offset + length, vi->tree_params.tree_size);
+	if (offset >= end_offset)
+		return 0;
+	offs_in_page = offset_in_page(offset);
+	last_index = (end_offset - 1) >> PAGE_SHIFT;
+
+	/*
+	 * Iterate through each Merkle tree page in the requested range and copy
+	 * the requested portion to userspace.  Note that the Merkle tree block
+	 * size isn't important here, as we are returning a byte stream; i.e.,
+	 * we can just work with pages even if the tree block size != PAGE_SIZE.
+	 */
+	for (index = offset >> PAGE_SHIFT; index <= last_index; index++) {
+		unsigned long num_ra_pages =
+			min_t(unsigned long, last_index - index + 1,
+			      inode->i_sb->s_bdi->io_pages);
+		unsigned int bytes_to_copy = min_t(u64, end_offset - offset,
+						   PAGE_SIZE - offs_in_page);
+		struct page *page;
+		const void *virt;
+
+		page = vops->read_merkle_tree_page(inode, index, num_ra_pages);
+		if (IS_ERR(page)) {
+			err = PTR_ERR(page);
+			fsverity_err(inode,
+				     "Error %d reading Merkle tree page %lu",
+				     err, index);
+			break;
+		}
+
+		virt = kmap(page);
+		if (copy_to_user(buf, virt + offs_in_page, bytes_to_copy)) {
+			kunmap(page);
+			put_page(page);
+			err = -EFAULT;
+			break;
+		}
+		kunmap(page);
+		put_page(page);
+
+		retval += bytes_to_copy;
+		buf += bytes_to_copy;
+		offset += bytes_to_copy;
+
+		if (fatal_signal_pending(current))  {
+			err = -EINTR;
+			break;
+		}
+		cond_resched();
+		offs_in_page = 0;
+	}
+	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;
+}
+
+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
+ * @uarg: user pointer to fsverity_read_metadata_arg
+ *
+ * Return: length read on success, 0 on EOF, -errno on failure
+ */
+int fsverity_ioctl_read_metadata(struct file *filp, const void __user *uarg)
+{
+	struct inode *inode = file_inode(filp);
+	const struct fsverity_info *vi;
+	struct fsverity_read_metadata_arg arg;
+	int length;
+	void __user *buf;
+
+	vi = fsverity_get_info(inode);
+	if (!vi)
+		return -ENODATA; /* not a verity file */
+	/*
+	 * Note that we don't have to explicitly check that the file is open for
+	 * reading, since verity files can only be opened for reading.
+	 */
+
+	if (copy_from_user(&arg, uarg, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.__reserved)
+		return -EINVAL;
+
+	/* offset + length must not overflow. */
+	if (arg.offset + arg.length < arg.offset)
+		return -EINVAL;
+
+	/* Ensure that the return value will fit in INT_MAX. */
+	length = min_t(u64, arg.length, INT_MAX);
+
+	buf = u64_to_user_ptr(arg.buf_ptr);
+
+	switch (arg.metadata_type) {
+	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);
+	case FS_VERITY_METADATA_TYPE_SIGNATURE:
+		return fsverity_read_signature(inode, buf, arg.offset, length);
+	default:
+		return -EINVAL;
+	}
+}
+EXPORT_SYMBOL_GPL(fsverity_ioctl_read_metadata);

fs/verity/signature.c

@@ -29,21 +29,19 @@ static struct key *fsverity_keyring;
 /**
  * fsverity_verify_signature() - check a verity file's signature
  * @vi: the file's fsverity_info
- * @desc: the file's fsverity_descriptor
- * @desc_size: size of @desc
+ * @signature: the file's built-in signature
+ * @sig_size: size of signature in bytes, or 0 if no signature
  *
- * If the file's fs-verity descriptor includes a signature of the file digest,
- * verify it against the certificates in the fs-verity keyring.
+ * If the file includes a signature of its fs-verity file digest, verify it
+ * against the certificates in the fs-verity keyring.
  *
  * Return: 0 on success (signature valid or not required); -errno on failure
  */
 int fsverity_verify_signature(const struct fsverity_info *vi,
-			      const struct fsverity_descriptor *desc,
-			      size_t desc_size)
+			      const u8 *signature, size_t sig_size)
 {
 	const struct inode *inode = vi->inode;
 	const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
-	const u32 sig_size = le32_to_cpu(desc->sig_size);
 	struct fsverity_formatted_digest *d;
 	int err;
 
@@ -56,11 +54,6 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 		return 0;
 	}
 
-	if (sig_size > desc_size - sizeof(*desc)) {
-		fsverity_err(inode, "Signature overflows verity descriptor");
-		return -EBADMSG;
-	}
-
 	d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
 	if (!d)
 		return -ENOMEM;
@@ -70,8 +63,7 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 	memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
 
 	err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
-				     desc->signature, sig_size,
-				     fsverity_keyring,
+				     signature, sig_size, fsverity_keyring,
 				     VERIFYING_UNSPECIFIED_SIGNATURE,
 				     NULL, NULL);
 	kfree(d);

include/linux/fsverity.h

@@ -138,6 +138,10 @@ int fsverity_file_open(struct inode *inode, struct file *filp);
 int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
 void fsverity_cleanup_inode(struct inode *inode);
 
+/* read_metadata.c */
+
+int fsverity_ioctl_read_metadata(struct file *filp, const void __user *uarg);
+
 /* verify.c */
 
 bool fsverity_verify_page(struct page *page);
@@ -183,6 +187,14 @@ static inline void fsverity_cleanup_inode(struct inode *inode)
 {
 }
 
+/* read_metadata.c */
+
+static inline int fsverity_ioctl_read_metadata(struct file *filp,
+					       const void __user *uarg)
+{
+	return -EOPNOTSUPP;
+}
+
 /* verify.c */
 
 static inline bool fsverity_verify_page(struct page *page)

include/uapi/linux/fsverity.h

@@ -83,7 +83,21 @@ struct fsverity_formatted_digest {
 	__u8 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;
+	__u64 offset;
+	__u64 length;
+	__u64 buf_ptr;
+	__u64 __reserved;
+};
+
 #define FS_IOC_ENABLE_VERITY	_IOW('f', 133, struct fsverity_enable_arg)
 #define FS_IOC_MEASURE_VERITY	_IOWR('f', 134, struct fsverity_digest)
+#define FS_IOC_READ_VERITY_METADATA \
+	_IOWR('f', 135, struct fsverity_read_metadata_arg)
 
 #endif /* _UAPI_LINUX_FSVERITY_H */