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

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Date:   Thu, 19 Aug 2021 10:12:43 -0400
From:   Dennis Zhou <dennis@...nel.org>
To:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Dennis Zhou <dennis@...nel.org>, linux-kernel@...r.kernel.org,
        Daniel Micay <danielmicay@...il.com>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        linux-mm@...ck.org, Joe Perches <joe@...ches.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Whitcroft <apw@...onical.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        clang-built-linux@...glegroups.com, linux-kbuild@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 6/7] percpu: Add __alloc_size attributes for better
 bounds checking

On Wed, Aug 18, 2021 at 08:36:50PM -0700, Kees Cook wrote:
> 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:
> 

I see that this is already pulled by Andrew, but I think it would be
good to modify the commit log to add a short bit about this limitation.
Otherwise, the commit reads as if it's doing way more than it is.

Thanks,
Dennis

> 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@...il.com>
> > > Signed-off-by: Daniel Micay <danielmicay@...il.com>
> > > Cc: Dennis Zhou <dennis@...nel.org>
> > > Cc: Tejun Heo <tj@...nel.org>
> > > Cc: Christoph Lameter <cl@...ux.com>
> > > Cc: linux-mm@...ck.org
> > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > ---
> > >  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