Re: [[PATCH v3]] xen/guest_access: Harden *copy_to_guest_offset() to prevent const dest operand

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 17 Apr 2020, Julien Grall wrote:
> On 16/04/2020 13:19, Jan Beulich wrote:
> > On 16.04.2020 13:24, Julien Grall wrote:
> > > From: Julien Grall <jgrall@xxxxxxxxxx>
> > > 
> > > At the moment, *copy_to_guest_offset() will allow the hypervisor to copy
> > > data to guest handle marked const.
> > > 
> > > Thankfully, no users of the helper will do that. Rather than hoping this
> > > can be caught during review, harden copy_to_guest_offset() so the build
> > > will fail if such users are introduced.
> > > 
> > > There is no easy way to check whether a const is NULL in C99. The
> > > approach used is to introduce an unused variable that is non-const and
> > > assign the handle. If the handle were const, this would fail at build
> > > because without an explicit cast, it is not possible to assign a const
> > > variable to a non-const variable.
> > > 
> > > Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> > 
> > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > with one further remark:
> > 
> > > --- a/xen/include/asm-x86/guest_access.h
> > > +++ b/xen/include/asm-x86/guest_access.h
> > > @@ -87,6 +87,8 @@
> > >   #define copy_to_guest_offset(hnd, off, ptr, nr) ({      \
> > >       const typeof(*(ptr)) *_s = (ptr);                   \
> > >       char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
> > > +    /* Check if the handle is not const */              \
> > > +    void *__maybe_unused _t = (hnd).p;                  \
> > 
> > Not being a native speaker, to me "if" doesn't look appropriate
> > here. I'd use "that" instead, but you may want to confirm this.
> > Overall then maybe "Check that the handle is not for a const type"?
> 
> I am happy with the new suggestion. I will fixup while comitting it.
> 
> I would also need a review from Stefano here also.

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>