linux-kernel - Re: [PATCH] HID: usbhid: fix recurrent out-of-bounds bug in usbhid

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Date: Tue, 4 Jun 2024 10:21:15 -0700
From: Kees Cook <kees@...nel.org>
To: Nikita Zhandarovich <n.zhandarovich@...tech.ru>
Cc: Jiri Kosina <jikos@...nel.org>, Benjamin Tissoires <bentiss@...nel.org>,
	linux-usb@...r.kernel.org, linux-input@...r.kernel.org,
	syzkaller-bugs@...glegroups.com, linux-kernel@...r.kernel.org,
	syzbot+c52569baf0c843f35495@...kaller.appspotmail.com,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] HID: usbhid: fix recurrent out-of-bounds bug in
 usbhid_parse()

On Tue, Jun 04, 2024 at 10:09:43AM -0700, Nikita Zhandarovich wrote:
> Hi,
> 
> On 6/4/24 07:15, Jiri Kosina wrote:
> > On Tue, 4 Jun 2024, Kees Cook wrote:
> > 
> >> This isn't the right solution. The problem is that hid_class_descriptor 
> >> is a flexible array but was sized as a single element fake flexible 
> >> array:
> >>
> >> struct hid_descriptor {
> >> 	   __u8  bLength;
> >> 	   __u8  bDescriptorType;
> >> 	   __le16 bcdHID;
> >> 	   __u8  bCountryCode;
> >> 	   __u8  bNumDescriptors;
> >>
> >> 	   struct hid_class_descriptor desc[1];
> >> } __attribute__ ((packed));
> >>
> >> This likely needs to be: 
> >>
> >> struct hid_class_descriptor desc[] __counted_by(bNumDescriptors);
> >>
> >> And then check for any sizeof() uses of the struct that might have changed.
> > 
> > Ah, you are of course right, not sure what I was thinking. Thanks a lot 
> > for catching my brainfart.
> > 
> > I am dropping the patch for now; Nikita, will you please send a refreshed 
> > one?
> > 
> 
> Thanks for catching my mistake.
> 
> I'll gladly send a revised version, hoping to do it very soon.

I spent a little more time looking at this, and I'm not sure I
understand where the actual space for the descriptors comes from?
There's interface->extra that is being parsed, and effectively
hid_descriptor is being mapped into it, but it uses "sizeof(struct
hid_descriptor)" for the limit. Is more than 1 descriptor expected to
work correctly? Or is the limit being ignored? I'm a bit confused by
this code...

-- 
Kees Cook