fs-verity: add the hook for file ->open()

Add the fsverity_file_open() function, which prepares an fs-verity file
to be read from.  If not already done, it loads the fs-verity descriptor
from the filesystem and sets up an fsverity_info structure for the inode
which describes the Merkle tree and contains the file measurement.  It
also denies all attempts to open verity files for writing.

This commit also begins the include/linux/fsverity.h header, which
declares the interface between fs/verity/ and filesystems.

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

1 files changed, 71 insertions, 0 deletions

diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
new file mode 100644
index 000000000000..09b04dab6452
--- /dev/null
+++ b/include/linux/fsverity.h
@@ -0,0 +1,71 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * fs-verity: read-only file-based authenticity protection
+ *
+ * This header declares the interface between the fs/verity/ support layer and
+ * filesystems that support fs-verity.
+ *
+ * Copyright 2019 Google LLC
+ */
+
+#ifndef _LINUX_FSVERITY_H
+#define _LINUX_FSVERITY_H
+
+#include <linux/fs.h>
+#include <uapi/linux/fsverity.h>
+
+/* Verity operations for filesystems */
+struct fsverity_operations {
+
+	/**
+	 * Get the verity descriptor of the given inode.
+	 *
+	 * @inode: an inode with the S_VERITY flag set
+	 * @buf: buffer in which to place the verity descriptor
+	 * @bufsize: size of @buf, or 0 to retrieve the size only
+	 *
+	 * If bufsize == 0, then the size of the verity descriptor is returned.
+	 * Otherwise the verity descriptor is written to 'buf' and its actual
+	 * size is returned; -ERANGE is returned if it's too large.  This may be
+	 * called by multiple processes concurrently on the same inode.
+	 *
+	 * Return: the size on success, -errno on failure
+	 */
+	int (*get_verity_descriptor)(struct inode *inode, void *buf,
+				     size_t bufsize);
+};
+
+#ifdef CONFIG_FS_VERITY
+
+static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
+{
+	/* pairs with the cmpxchg() in fsverity_set_info() */
+	return READ_ONCE(inode->i_verity_info);
+}
+
+/* open.c */
+
+extern int fsverity_file_open(struct inode *inode, struct file *filp);
+extern void fsverity_cleanup_inode(struct inode *inode);
+
+#else /* !CONFIG_FS_VERITY */
+
+static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
+{
+	return NULL;
+}
+
+/* open.c */
+
+static inline int fsverity_file_open(struct inode *inode, struct file *filp)
+{
+	return IS_VERITY(inode) ? -EOPNOTSUPP : 0;
+}
+
+static inline void fsverity_cleanup_inode(struct inode *inode)
+{
+}
+
+#endif	/* !CONFIG_FS_VERITY */
+
+#endif	/* _LINUX_FSVERITY_H */