On Tue, Jan 28, 2025 at 12:00:41PM -0500, Alan Stern wrote:

On Tue, Jan 28, 2025 at 05:45:21AM -0800, Nikita Zhandarovich wrote:

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Hello,

On 6/4/24 10:45, Alan Stern wrote:

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On Tue, Jun 04, 2024 at 10:21:15AM -0700, Kees Cook wrote:

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On Tue, Jun 04, 2024 at 10:09:43AM -0700, Nikita Zhandarovich wrote:

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Hi,

On 6/4/24 07:15, Jiri Kosina wrote:

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On Tue, 4 Jun 2024, Kees Cook wrote:

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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.

Alan, I finally got around to preparing a revised version of the
required patch and encountered a few issues. I could use some advice in
this matter...

If we change 'struct hid_descriptor' as you suggested,

I didn't make that suggestion.  Kees Cook did.

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 which does make
sense, most occurrences of that type are easy enough to fix.

1) usbhid_parse() starts working properly if there are more than 1
descriptors, sizeof(struct hid_descriptor) may be turned into something
crude but straightforward like sizeof(struct hid_descriptor) +
sizeof(struct hid_class_descriptor).

2) 'hid_descriptor' in drivers/hid/hid-hyperv.c remains innocuous as
well as only 1 descriptor expected there. My impression is only some
small changes are needed there.

However, the issue that stumps me is the following: static struct
hid_descriptor hidg_desc in drivers/usb/gadget/function/f_hid.c relies
on a static nature of that one descriptor. hidg_desc ends up being used
elsewhere, in other static structures. Basically, using __counted_by
requires a lot of changes, as I see it, out of scope of merely closing
an UBSAN error.

The hidg_desc structure needs to contain room for a single 
hid_descriptor containing a single hid_class_descriptor.  I think you 
can define it that way by doing something like this:

static struct hid_descriptor hidg_desc = {
	.bLength			= sizeof hidg_desc,
	.bDescriptorType		= HID_DT_HID,
	.bcdHID				= cpu_to_le16(0x0101),
	.bCountryCode			= 0x00,
	.bNumDescriptors		= 0x1,
	.desc				= {
		{
			.bDescriptorType	= 0, /* DYNAMIC */
			.wDescriptorLength	= 0, /* DYNAMIC */
		}
	}
};

Or maybe it needs to be:

	.desc				= { {0, 0} } /* DYNAMIC */

I'm not sure what is the correct syntax; you'll have to figure that out.  

Either should work.

You'll have to be more careful about the definition of hidg_desc_copy in 
hidg_setup(), however.  You might want to define hidg_desc_copy as an 
alias to the start of a byte array of the right size.

For an on-stack fixed-size flex array structure, you can use:

	DEFINE_FLEX(struct hid_descriptor, hidg_desc_copy,
		    desc, bNumDescriptors, 1);
	*hidg_desc_copy = hidg_desc;

and then adjust the "hidg_desc_copy." instances to "hidg_desc_copy->"

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Is this approach still worthy pursuing or should I look into some neater
solution?

I think you should persist with this approach.

Alan Stern

-- 
Kees Cook