Re: [PATCH v2 6/7] percpu: Add __alloc_size attributes for better bounds checking

[Kees Cook]

On Wed, Aug 18, 2021 at 08:42:59PM -0400, Dennis Zhou wrote:
> On Wed, Aug 18, 2021 at 02:40:20PM -0700, Kees Cook wrote:
> > As already done in GrapheneOS, add the __alloc_size attribute for
> > appropriate percpu allocator interfaces, to provide additional hinting
> > for better bounds checking, assisting CONFIG_FORTIFY_SOURCE and other
> > compiler optimizations.
> 
> Can you elaborate a little bit for me how this works for percpu? In any
> case that's not uniprocessor, any modification is done through address
> accessors and not on the returned percpu pointer. Is the metadata kept
> by gcc/clang able to transpire the percpu pointer accessors?

That's an excellent point. :P I haven't tested it through the accessors,
but I guess it's possible that this is only useful for UP, and even
then, only where the access is very close to the "allocation", maybe
like:

char __percpu *test_buf;

	char *buf;
	test_var = __alloc_percpu(16, __alignof__(char));
	buf = per_cpu_ptr(test_buf, get_cpu());
	...
	buf[20] = '!';

-Kees

> 
> Thanks,
> Dennis
> 
> > 
> > Co-developed-by: Daniel Micay <danielmicay@xxxxxxxxx>
> > Signed-off-by: Daniel Micay <danielmicay@xxxxxxxxx>
> > Cc: Dennis Zhou <dennis@xxxxxxxxxx>
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  include/linux/percpu.h | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> > index 5e76af742c80..119f41815b32 100644
> > --- a/include/linux/percpu.h
> > +++ b/include/linux/percpu.h
> > @@ -123,6 +123,7 @@ extern int __init pcpu_page_first_chunk(size_t reserved_size,
> >  				pcpu_fc_populate_pte_fn_t populate_pte_fn);
> >  #endif
> >  
> > +__alloc_size(1)
> >  extern void __percpu *__alloc_reserved_percpu(size_t size, size_t align);
> >  extern bool __is_kernel_percpu_address(unsigned long addr, unsigned long *can_addr);
> >  extern bool is_kernel_percpu_address(unsigned long addr);
> > @@ -131,7 +132,9 @@ extern bool is_kernel_percpu_address(unsigned long addr);
> >  extern void __init setup_per_cpu_areas(void);
> >  #endif
> >  
> > +__alloc_size(1)
> >  extern void __percpu *__alloc_percpu_gfp(size_t size, size_t align, gfp_t gfp);
> > +__alloc_size(1)
> >  extern void __percpu *__alloc_percpu(size_t size, size_t align);
> >  extern void free_percpu(void __percpu *__pdata);
> >  extern phys_addr_t per_cpu_ptr_to_phys(void *addr);
> > -- 
> > 2.30.2
> > 

-- 
Kees Cook