On Tue, Jun 04, 2024 at 10:21:15AM -0700, Kees Cook wrote:
> 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.
That's a lower limit, not an upper limit. The hid_descriptor must
include at least one hid_class_descriptor, but it can include more.
That's what the min_t() calculation of num_descriptors is meant to
figure out.
> Is more than 1 descriptor expected to
> work correctly?
More than one hid_class_descriptor -- yes.
> Or is the limit being ignored? I'm a bit confused by
> this code...
Does this explain it?
Alan Stern
