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

Add a function fsverity_prepare_setattr() which filesystems that support
fs-verity must call to deny truncates of verity files.

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, 21 insertions, 0 deletions

diff --git a/fs/verity/open.c b/fs/verity/open.c
index 8013f77f907e..2cb2fe8082bf 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -296,6 +296,27 @@ int fsverity_file_open(struct inode *inode, struct file *filp)
 EXPORT_SYMBOL_GPL(fsverity_file_open);
 
 /**
+ * fsverity_prepare_setattr() - prepare to change a verity inode's attributes
+ * @dentry: dentry through which the inode is being changed
+ * @attr: attributes to change
+ *
+ * Verity files are immutable, so deny truncates.  This isn't covered by the
+ * open-time check because sys_truncate() takes a path, not a file descriptor.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
+{
+	if (IS_VERITY(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE)) {
+		pr_debug("Denying truncate of verity file (ino %lu)\n",
+			 d_inode(dentry)->i_ino);
+		return -EPERM;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fsverity_prepare_setattr);
+
+/**
  * fsverity_cleanup_inode() - free the inode's verity info, if present
  *
  * Filesystems must call this on inode eviction to free ->i_verity_info.