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

[Jiri Kosina <jikos@kernel.org>]

On Wed, 11 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?

As far as I remember, this is just very lousy way of properly rounding the 
size up (see 27ce405039bfe). So I believe HID_MAX_BUFFER_SIZE -1 is still 
functionally correct.

Thanks,

-- 
Jiri Kosina
SUSE Labs