Re: [PATCH 1/4] counter: Adjust final parameter type in function and signal callbacks

Nathan Chancellor
2022-11-02T20:23:54Z

On Wed, Nov 02, 2022 at 12:21:23PM -0700, Kees Cook wrote:
> On Wed, Nov 02, 2022 at 10:22:14AM -0700, Nathan Chancellor wrote:
> > The ->signal_u32_read(), ->count_u32_read(), and ->count_u32_write()
> > callbacks in 'struct counter_comp' expect the final parameter to have a
> > type of 'u32' or 'u32 *' but the ops functions that are being assigned
> > to those callbacks have an enumerated type as the final parameter. While
> > these are compatible from an ABI perspective, they will fail the
> > aforementioned CFI checks.
> >
> > Adjust the type of the final parameter in the ->signal_read(),
> > ->function_read(), and ->function_write() callbacks in 'struct
> > counter_ops' and their implementations to match the prototypes in
> > 'struct counter_comp' to clear up these warnings and CFI failures.
>
> I don't understand these changes. Where do 'struct counter_comp'
> and 'struct counter_ops' get confused? I can only find matching
> ops/assignments/calls, so I must be missing something. This looks like
> a loss of CFI granularity instead of having wrappers added if there is
> an enum/u32 conversion needed somewhere.

Right, I am not the biggest fan of this change myself and it is entirely
possible that I am misreading the warnings from the commit message but I
do not see how

comp_node.comp.signal_u32_read = counter->ops->signal_read;

and

comp_node.comp.count_u32_read = counter->ops->function_read;

in counter_add_watch(),

comp.signal_u32_read = counter->ops->signal_read;

in counter_signal_attrs_create(), and

comp.count_u32_read = counter->ops->function_read;
comp.count_u32_write = counter->ops->function_write;

in counter_count_attrs_create() are currently safe under kCFI, since the
final parameter type of the prototypes in 'struct counter_ops' does not
match the final parameter type of the prototypes in 'struct
counter_comp'. I would expect the indirect calls in counter_get_data()
and counter_comp_u32_show() to fail currently.

I briefly looked at making the 'struct counter_comp' callbacks match the
'struct counter_ops' ones but the COUNTER_COMP macros in
include/linux/counter.h made it seem like these callbacks might be used
by implementations that might use different enumerated types as the
final parameter. I can look a little closer to see if we can make
everything match.

I am not sure how wrappers would work here, I can take a look into how
feasible that is.

Cheers,
Nathan