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From: Peilin Ye <>
To: Dan Carpenter <>
Cc:, Jiri Kosina <>,,,
	Benjamin Tissoires <>,,
Subject: Re: [Linux-kernel-mentees] [PATCH v1] usbhid: Fix slab-out-of-bounds write in hiddev_ioctl_usage()
Date: Mon, 20 Jul 2020 15:05:08 -0400	[thread overview]
Message-ID: <20200720190508.GA1946@PWN> (raw)
In-Reply-To: <20200720121257.GJ2571@kadam>

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_parser_main() calls different functions in order to process
different type of items:


	static int (*dispatch_type[])(struct hid_parser *parser,
				      struct hid_item *item) = {

In this case, hid_parser_main() calls hid_add_field(), which in turn
calls hid_register_field(), which allocates the `field` object as you


	field = kzalloc((sizeof(struct hid_field) +
			 usages * sizeof(struct hid_usage) +
			 values * sizeof(unsigned)), GFP_KERNEL);

Here, `values` equals to `global.report_count`. See how it is being


	field = hid_register_field(report, usages, parser->global.report_count);

In hid_parser_main(), `global.report_count` can be set by calling

However, the syzkaller reproducer made hid_parser_main() to call
hid_parser_global() __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.

Apparently hid_parser_main() doesn't care about which item (main, local,
global and reserved) gets processed first. I am new to this code and I
don't know whether this is by design, but this arbitrarity is
apparently causing some issues.

As another example, in hid_add_field():


	report->size += parser->global.report_size * parser->global.report_count;

If `global.report_count` is zero, `report->size` gets increased by zero.
Is this working as intended? It seems weird to me.

Thank you,

Peilin Ye
Linux-kernel-mentees mailing list

  reply	other threads:[~2020-07-20 19:05 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-18 23:12 [Linux-kernel-mentees] [PATCH v1] usbhid: Fix slab-out-of-bounds write in hiddev_ioctl_usage() Peilin Ye
2020-07-20 11:54 ` Dan Carpenter
2020-07-20 12:12   ` Dan Carpenter
2020-07-20 19:05     ` Peilin Ye [this message]
2020-07-20 19:16     ` Peilin Ye
2020-07-21  7:16       ` Dan Carpenter
2020-07-21  8:27         ` Greg Kroah-Hartman
2020-07-21  8:39           ` Dan Carpenter
2020-07-21  8:39         ` Peilin Ye
2020-07-20 19:52 ` [Linux-kernel-mentees] [PATCH v2] " Peilin Ye
2020-07-23 14:51   ` Dan Carpenter
2020-07-29 11:37 ` [Linux-kernel-mentees] [PATCH v2 RESEND] " Peilin Ye
2020-07-29 14:35   ` Dan Carpenter
2020-08-17 10:21   ` Jiri Kosina
2020-08-18 10:00     ` Peilin Ye

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