fs-verity: implement FS_IOC_MEASURE_VERITY ioctl

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.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/verity/Makefile b/fs/verity/Makefile
index 04b3747..6f7675a 100644 (file)
--- 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 (file)
index 0000000..05049b6
--- /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 d1a5dbf..3b6b8cc 100644 (file)
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -118,6 +118,10 @@ 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);
@@ -145,6 +149,13 @@ 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)