Re: [PATCH 5.4 17/78] HID: Fix slab-out-of-bounds read in hid_field_extract (Broken!)

[Greg Kroah-Hartman]

On Wed, Feb 05, 2020 at 08:12:27AM +0100, peter enderborg wrote:
> On 1/14/20 11:00 AM, Greg Kroah-Hartman wrote:
> > From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> >
> > commit 8ec321e96e056de84022c032ffea253431a83c3c upstream.
> >
> > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > CC: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> >
> > ---
> >  drivers/hid/hid-core.c |    6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > --- a/drivers/hid/hid-core.c
> > +++ b/drivers/hid/hid-core.c
> > @@ -288,6 +288,12 @@ static int hid_add_field(struct hid_pars
> >  	offset = report->size;
> >  	report->size += parser->global.report_size * parser->global.report_count;
> >  
> > +	/* Total size check: Allow for possible report index byte */
> > +	if (report->size > (HID_MAX_BUFFER_SIZE - 1) << 3) {
> > +		hid_err(parser->device, "report is too long\n");
> > +		return -1;
> > +	}
> > +
> >  	if (!parser->local.usage_index) /* Ignore padding fields */
> >  		return 0;
> >  
> >
> >
> >
> This patch breaks Elgato StreamDeck.

Does that mean the device is broken with a too-large of a report?

Is it broken in Linus's tree?  If so, can you work with the HID
developers to fix it there so we can backport the fix to all stable
trees?

thanks,

greg k-h