[PATCH v3 07/15] fs-verity: add the hook for file ->setattr()

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

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

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/verity/open.c         | 21 +++++++++++++++++++++
 include/linux/fsverity.h |  7 +++++++
 2 files changed, 28 insertions(+)

diff --git a/fs/verity/open.c b/fs/verity/open.c
index e6d73d73b000d..f779a150b4ede 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -292,6 +292,27 @@ 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
  *
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 3b60195e3c51a..29bb4d007ad16 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -40,6 +40,7 @@ struct fsverity_operations {
 /* open.c */
 
 extern int fsverity_file_open(struct inode *inode, struct file *filp);
+extern int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
 extern void fsverity_cleanup_inode(struct inode *inode);
 
 #else /* !CONFIG_FS_VERITY */
@@ -51,6 +52,12 @@ static inline int fsverity_file_open(struct inode *inode, struct file *filp)
 	return IS_VERITY(inode) ? -EOPNOTSUPP : 0;
 }
 
+static inline int fsverity_prepare_setattr(struct dentry *dentry,
+					   struct iattr *attr)
+{
+	return IS_VERITY(d_inode(dentry)) ? -EOPNOTSUPP : 0;
+}
+
 static inline void fsverity_cleanup_inode(struct inode *inode)
 {
 }
-- 
2.21.0