[PATCH] debugfs: Fix NULL pointer dereference at debugfs_read_file_str

[yangshiguang1011]

From: yangshiguang <yangshiguang@xxxxxxxxxx>

Check in debugfs_read_file_str() if the string pointer is NULL.

When creating a node using debugfs_create_str(), the string parameter
value can be NULL to indicate empty/unused/ignored.
However, reading this node using debugfs_read_file_str() will cause a
kernel panic.
This should not be fatal, so return an invalid error.

Signed-off-by: yangshiguang <yangshiguang@xxxxxxxxxx>
Fixes: 9af0440ec86e ("debugfs: Implement debugfs_create_str()")
Cc: stable@xxxxxxxxxxxxxxx
---
 fs/debugfs/file.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 3ec3324c2060..a22ff0ceb230 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -1026,6 +1026,9 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
 		return ret;
 
 	str = *(char **)file->private_data;
+	if (!str)
+		return -EINVAL;
+
 	len = strlen(str) + 1;
 	copy = kmalloc(len, GFP_KERNEL);
 	if (!copy) {
-- 
2.43.0