[PATCH v2 3/3] debugfs: make debugfs_create_str() read-only

debugfs_create_str() supports replacing the backing string from userspace.
Concurrent writers can race and free the same old string twice.

All writable in-tree users have been converted to local file operations.
Remove the generic write support from debugfs_create_str(), and refuse to
create a file when the caller passes write permission bits.

This makes unsupported writable use visible instead of silently creating a
file with different permissions.

Fixes: 86b5488121db ("debugfs: Add write support to debugfs_create_str()")
Signed-off-by: Yichong Chen <[email protected]>
---
 fs/debugfs/file.c | 81 ++++++-----------------------------------------
 1 file changed, 10 insertions(+), 71 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 08de6652a4f3..f5abd067b260 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -1049,98 +1049,37 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
 	return ret;
 }
 
-static ssize_t debugfs_write_file_str(struct file *file, const char __user *user_buf,
-				      size_t count, loff_t *ppos)
-{
-	struct dentry *dentry = F_DENTRY(file);
-	char *old, *new = NULL;
-	int pos = *ppos;
-	int r;
-
-	r = debugfs_file_get(dentry);
-	if (unlikely(r))
-		return r;
-
-	old = *(char **)file->private_data;
-
-	/* only allow strict concatenation */
-	r = -EINVAL;
-	if (pos && pos != strlen(old))
-		goto error;
-
-	r = -E2BIG;
-	if (pos + count + 1 > PAGE_SIZE)
-		goto error;
-
-	r = -ENOMEM;
-	new = kmalloc(pos + count + 1, GFP_KERNEL);
-	if (!new)
-		goto error;
-
-	if (pos)
-		memcpy(new, old, pos);
-
-	r = -EFAULT;
-	if (copy_from_user(new + pos, user_buf, count))
-		goto error;
-
-	new[pos + count] = '\0';
-	strim(new);
-
-	rcu_assign_pointer(*(char __rcu **)file->private_data, new);
-	synchronize_rcu();
-	kfree(old);
-
-	debugfs_file_put(dentry);
-	return count;
-
-error:
-	kfree(new);
-	debugfs_file_put(dentry);
-	return r;
-}
-
 static const struct file_operations fops_str = {
 	.read =		debugfs_read_file_str,
-	.write =	debugfs_write_file_str,
-	.open =		simple_open,
-	.llseek =	default_llseek,
-};
-
-static const struct file_operations fops_str_ro = {
-	.read =		debugfs_read_file_str,
-	.open =		simple_open,
-	.llseek =	default_llseek,
-};
-
-static const struct file_operations fops_str_wo = {
-	.write =	debugfs_write_file_str,
 	.open =		simple_open,
 	.llseek =	default_llseek,
 };
 
 /**
- * debugfs_create_str - create a debugfs file that is used to read and write a string value
+ * debugfs_create_str - create a debugfs file that is used to read a string value
  * @name: a pointer to a string containing the name of the file to create.
  * @mode: the permission that the file should have
  * @parent: a pointer to the parent dentry for this file.  This should be a
  *          directory dentry if set.  If this parameter is %NULL, then the
  *          file will be created in the root of the debugfs filesystem.
- * @value: a pointer to the variable that the file should read to and write
- *         from. This pointer and the string it points to must not be %NULL.
+ * @value: a pointer to the variable that the file should read from. This
+ *         pointer and the string it points to must not be %NULL.
  *
  * This function creates a file in debugfs with the given name that
- * contains the value of the variable @value.  If the @mode variable is so
- * set, it can be read from, and written to.
+ * contains the value of the variable @value.  The file can be read from.
+ * Writable files are not supported; if @mode contains write permission bits,
+ * no file is created.
  */
 void debugfs_create_str(const char *name, umode_t mode,
 			struct dentry *parent, char **value)
 {
 	if (WARN_ON(!value || !*value))
 		return;
+	if (WARN_ONCE(mode & 0222,
+		      "%s() does not support writable files\n", __func__))
+		return;
 
-	debugfs_create_mode_unsafe(name, mode, parent, value, &fops_str,
-				   &fops_str_ro, &fops_str_wo);
+	debugfs_create_file_unsafe(name, mode, parent, value, &fops_str);
 }
 EXPORT_SYMBOL_GPL(debugfs_create_str);
 
-- 
2.51.0