Re: [Linux-kernel-mentees] [PATCH v1] usbhid: Fix slab-out-of-bounds write in hiddev_ioctl_usage()

[Dan Carpenter]

For some reason the reply-to header on your email is bogus:

Reply-To: 20200720121257.GJ2571@kadam

"kadam" is a system on my home network.

On Mon, Jul 20, 2020 at 03:16:56PM -0400, Peilin Ye wrote:
> I made some mistakes in the previous e-mail. Please ignore that. There
> are a lot of things going on...Sorry for that.
> 
> On Mon, Jul 20, 2020 at 03:12:57PM +0300, Dan Carpenter wrote:
> > So another option would be to just add HIDIOCGUSAGE and HIDIOCSUSAGE to
> > the earlier check.  That risks breaking userspace.  Another option is to
> > just add a check like you did earlier to the HIDIOCGUSAGE case.
> > Probably just do option #2 and resend.
> 
> Sure, I will just add the same check to the HIDIOCGUSAGE case for the
> time being. Thank you for the detailed explanation.
> 
> Here's what I found after digging a bit further though:
> 
> hid_open_report() calls different functions in order to process
> different type of items:
> 
> drivers/hid/hid-core.c:1193:
> 
>         static int (*dispatch_type[])(struct hid_parser *parser,
>                                       struct hid_item *item) = {
>                 hid_parser_main,
>                 hid_parser_global,
>                 hid_parser_local,
>                 hid_parser_reserved
>         };
> 
> In this case, hid_parser_main() calls hid_add_field(), which in turn
> calls hid_register_field(), which allocates the `field` object as you
> mentioned:
> 
> drivers/hid/hid-core.c:102:
> 
>         field = kzalloc((sizeof(struct hid_field) +
>                          usages * sizeof(struct hid_usage) +
>                          values * sizeof(unsigned)), GFP_KERNEL);

Yeah.  And in the caller it does:

drivers/hid/hid-core.c
   297          if (!parser->local.usage_index) /* Ignore padding fields */
   298                  return 0;
   299  
   300          usages = max_t(unsigned, parser->local.usage_index,
                ^^^^^^^^^^^^^^
   301                                   parser->global.report_count);
   302  
   303          field = hid_register_field(report, usages, parser->global.report_count);
                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
So ->usages is always greater or equal to ->global.report_count.

   304          if (!field)
   305                  return 0;
   306  
   307          field->physical = hid_lookup_collection(parser, HID_COLLECTION_PHYSICAL);

> 
> Here, `values` equals to `global.report_count`. See how it is being
> called:
> 
> drivers/hid/hid-core.c:303:
> 
>         field = hid_register_field(report, usages, parser->global.report_count);
> 
> In hid_open_report(), `global.report_count` can be set by calling
> hid_parser_global().
> 
> However, the syzkaller reproducer made hid_open_report() to call
> hid_parser_main() __before__ `global.report_count` is properly set. It's
> zero. So hid_register_field() allocated `field` with `values` equals to
> zero - No room for value[] at all. I believe this caused the bug.

I don't know if zero is valid or not.  I suspect it is valid.  We have
no reason to think that it's invalid.

regards,
dan carpenter