Re: [PATCH] HID: Fix slab-out-of-bounds read in hid_field_extract

[Alan Stern <stern@rowland.harvard.edu>]

On Wed, 11 Dec 2019, Jiri Kosina wrote:

> On Tue, 10 Dec 2019, Alan Stern wrote:
> 
> > The syzbot fuzzer found a slab-out-of-bounds bug in the HID report
> > handler.  The bug was caused by a report descriptor which included a
> > field with size 12 bits and count 4899, for a total size of 7349
> > bytes.
> > 
> > The usbhid driver uses at most a single-page 4-KB buffer for reports.
> > In the test there wasn't any problem about overflowing the buffer,
> > since only one byte was received from the device.  Rather, the bug
> > occurred when the HID core tried to extract the data from the report
> > fields, which caused it to try reading data beyond the end of the
> > allocated buffer.
> > 
> > This patch fixes the problem by rejecting any report whose total
> > length exceeds the HID_MAX_BUFFER_SIZE limit (minus one byte to allow
> > for a possible report index).  In theory a device could have a report
> > longer than that, but if there was such a thing we wouldn't handle it 
> > correctly anyway.
> > 
> > Reported-and-tested-by: syzbot+09ef48aa58261464b621@syzkaller.appspotmail.com
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> > CC: <stable@vger.kernel.org>
> 
> Thanks for hunting this down Alan. Applied.

I just noticed this code:

u8 *hid_alloc_report_buf(struct hid_report *report, gfp_t flags)
{
	/*
	 * 7 extra bytes are necessary to achieve proper functionality
	 * of implement() working on 8 byte chunks
	 */

	u32 len = hid_report_len(report) + 7;

	return kmalloc(len, flags);
}

Does this indicate that the upper limit on a report length should 
really be HID_MAX_BUFFER_SIZE - 8 instead of HID_MAX_BUFFER_SIZE - 1?

Alan Stern