linux-ext4 - [PATCH v4 11/16] fs-verity: implement FS_IOC_MEASURE

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Date:   Thu,  6 Jun 2019 08:52:00 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-fscrypt@...r.kernel.org
Cc:     linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org, Jaegeuk Kim <jaegeuk@...nel.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Victor Hsieh <victorhsieh@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH v4 11/16] fs-verity: implement FS_IOC_MEASURE_VERITY ioctl

From: Eric Biggers <ebiggers@...gle.com>

Add a function for filesystems to call to implement the
FS_IOC_MEASURE_VERITY ioctl.  This ioctl retrieves the file measurement
that fs-verity calculated for the given file and is enforcing for reads;
i.e., reads that don't match this hash will fail.  This ioctl can be
used for authentication or logging of file measurements in userspace.

See the "FS_IOC_MEASURE_VERITY" section of
Documentation/filesystems/fsverity.rst for the documentation.

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/verity/Makefile       |  1 +
 fs/verity/measure.c      | 57 ++++++++++++++++++++++++++++++++++++++++
 include/linux/fsverity.h | 11 ++++++++
 3 files changed, 69 insertions(+)
 create mode 100644 fs/verity/measure.c

diff --git a/fs/verity/Makefile b/fs/verity/Makefile
index 04b37475fd28..6f7675ae0a31 100644
--- a/fs/verity/Makefile
+++ b/fs/verity/Makefile
@@ -3,5 +3,6 @@
 obj-$(CONFIG_FS_VERITY) += enable.o \
 			   hash_algs.o \
 			   init.o \
+			   measure.o \
 			   open.o \
 			   verify.o
diff --git a/fs/verity/measure.c b/fs/verity/measure.c
new file mode 100644
index 000000000000..05049b68c745
--- /dev/null
+++ b/fs/verity/measure.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fs/verity/measure.c: ioctl to get a verity file's measurement
+ *
+ * Copyright 2019 Google LLC
+ */
+
+#include "fsverity_private.h"
+
+#include <linux/uaccess.h>
+
+/**
+ * fsverity_ioctl_measure() - get a verity file's measurement
+ *
+ * Retrieve the file measurement that the kernel is enforcing for reads from a
+ * verity file.  See the "FS_IOC_MEASURE_VERITY" section of
+ * Documentation/filesystems/fsverity.rst for the documentation.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fsverity_ioctl_measure(struct file *filp, void __user *_uarg)
+{
+	const struct inode *inode = file_inode(filp);
+	struct fsverity_digest __user *uarg = _uarg;
+	const struct fsverity_info *vi;
+	const struct fsverity_hash_alg *hash_alg;
+	struct fsverity_digest arg;
+
+	vi = fsverity_get_info(inode);
+	if (!vi)
+		return -ENODATA; /* not a verity file */
+	hash_alg = vi->tree_params.hash_alg;
+
+	/*
+	 * The user specifies the digest_size their buffer has space for; we can
+	 * return the digest if it fits in the available space.  We write back
+	 * the actual size, which may be shorter than the user-specified size.
+	 */
+
+	if (get_user(arg.digest_size, &uarg->digest_size))
+		return -EFAULT;
+	if (arg.digest_size < hash_alg->digest_size)
+		return -EOVERFLOW;
+
+	memset(&arg, 0, sizeof(arg));
+	arg.digest_algorithm = hash_alg - fsverity_hash_algs;
+	arg.digest_size = hash_alg->digest_size;
+
+	if (copy_to_user(uarg, &arg, sizeof(arg)))
+		return -EFAULT;
+
+	if (copy_to_user(uarg->digest, vi->measurement, hash_alg->digest_size))
+		return -EFAULT;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fsverity_ioctl_measure);
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 7ef2ef826534..247359c86b72 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -116,6 +116,10 @@ static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
 
 extern int fsverity_ioctl_enable(struct file *filp, const void __user *arg);
 
+/* measure.c */
+
+extern int fsverity_ioctl_measure(struct file *filp, void __user *arg);
+
 /* open.c */
 
 extern int fsverity_file_open(struct inode *inode, struct file *filp);
@@ -143,6 +147,13 @@ static inline int fsverity_ioctl_enable(struct file *filp,
 	return -EOPNOTSUPP;
 }
 
+/* measure.c */
+
+static inline int fsverity_ioctl_measure(struct file *filp, void __user *arg)
+{
+	return -EOPNOTSUPP;
+}
+
 /* open.c */
 
 static inline int fsverity_file_open(struct inode *inode, struct file *filp)
-- 
2.21.0