[PATCH V3 1/6] VFS: Add a call to obtain a file's hash

[Matthew Garrett <matthewgarrett@google.com>]

From: Matthew Garrett <mjg59@google.com>

IMA wants to know what the hash of a file is, and currently does so by
reading the entire file and generating the hash. Some filesystems may
have the ability to store the hash in a secure manner resistant to
offline attacks (eg, filesystem-level file signing), and in that case
it's a performance win for IMA to be able to use that rather than having
to re-hash everything. This patch simply adds VFS-level support for
calling down to filesystems.

Signed-off-by: Matthew Garrett <mjg59@google.com>
---
 fs/read_write.c    | 24 ++++++++++++++++++++++++
 include/linux/fs.h |  6 +++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 61b43ad7608e..3ba7038ba9a9 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -2157,3 +2157,27 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
 	return ret;
 }
 EXPORT_SYMBOL(vfs_dedupe_file_range);
+
+/**
+ * vfs_gethash - obtain a file's hash
+ * @file:	file structure in question
+ * @hash_algo:	the hash algorithm requested
+ * @buf:	buffer to return the hash in
+ * @size:	size allocated for the buffer by the caller
+ *
+ * This function allows filesystems that support securely storing the hash
+ * of a file to return it rather than forcing the kernel to recalculate it.
+ * Filesystems that cannot provide guarantees about the hash being resistant
+ * to offline attack should not implement this functionality.
+ *
+ * Returns 0 on success, -EOPNOTSUPP if the filesystem doesn't support it.
+ */
+int vfs_get_hash(struct file *file, enum hash_algo hash, uint8_t *buf,
+		 size_t size)
+{
+	if (!file->f_op->get_hash)
+		return -EOPNOTSUPP;
+
+	return file->f_op->get_hash(file, hash, buf, size);
+}
+EXPORT_SYMBOL(vfs_get_hash);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index dd28e7679089..211f0e214953 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -43,6 +43,7 @@
 
 #include <asm/byteorder.h>
 #include <uapi/linux/fs.h>
+#include <uapi/linux/hash_info.h>
 
 struct backing_dev_info;
 struct bdi_writeback;
@@ -1822,6 +1823,8 @@ struct file_operations {
 				   struct file *file_out, loff_t pos_out,
 				   loff_t len, unsigned int remap_flags);
 	int (*fadvise)(struct file *, loff_t, loff_t, int);
+	int (*get_hash)(struct file *, enum hash_algo hash, uint8_t *buf,
+			size_t size);
 } __randomize_layout;
 
 struct inode_operations {
@@ -1898,7 +1901,8 @@ extern int vfs_dedupe_file_range(struct file *file,
 extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
 					struct file *dst_file, loff_t dst_pos,
 					loff_t len, unsigned int remap_flags);
-
+extern int vfs_get_hash(struct file *file, enum hash_algo hash, uint8_t *buf,
+			size_t size);
 
 struct super_operations {
    	struct inode *(*alloc_inode)(struct super_block *sb);
-- 
2.21.0.1020.gf2820cf01a-goog