usb: gadget: f_fs: fix NULL pointer dereference when there are no strings

commit f0688c8b81d2ea239c3fb0b848f623b579238d99 upstream.

If the descriptors do not need any strings and user space sends empty
set of strings, the ffs->stringtabs field remains NULL.  Thus
*ffs->stringtabs in functionfs_bind leads to a NULL pointer
dereferenece.

The bug was introduced by commit [fd7c9a007f: “use usb_string_ids_n()”].

While at it, remove double initialisation of lang local variable in
that function.

ffs->strings_count does not need to be checked in any way since in
the above scenario it will remain zero and usb_string_ids_n() is
a no-operation when colled with 0 argument.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
index 2b43343..5bcf7d0 100644 (file)
--- a/drivers/usb/gadget/f_fs.c
+++ b/drivers/usb/gadget/f_fs.c
@@ -1227,11 +1227,13 @@ static int functionfs_bind(struct ffs_data *ffs, struct usb_composite_dev *cdev)
        ffs->ep0req->context = ffs;
 
        lang = ffs->stringtabs;
-       for (lang = ffs->stringtabs; *lang; ++lang) {
-               struct usb_string *str = (*lang)->strings;
-               int id = first_id;
-               for (; str->s; ++id, ++str)
-                       str->id = id;
+       if (lang) {
+               for (; *lang; ++lang) {
+                       struct usb_string *str = (*lang)->strings;
+                       int id = first_id;
+                       for (; str->s; ++id, ++str)
+                               str->id = id;
+               }
        }
 
        ffs->gadget = cdev->gadget;