Re: [PATCH v3 4/8] slab: Add __alloc_size attributes for better bounds checking

[Kees Cook <keescook@chromium.org>]

On Wed, Oct 06, 2021 at 05:22:06AM +0200, Jann Horn wrote:
> On Wed, Oct 6, 2021 at 5:06 AM Kees Cook <keescook@chromium.org> wrote:
> > On Tue, Oct 05, 2021 at 06:47:17PM -0700, Andrew Morton wrote:
> > > On Thu, 30 Sep 2021 15:27:00 -0700 Kees Cook <keescook@chromium.org> wrote:
> > >
> > > > As already done in GrapheneOS, add the __alloc_size attribute for regular
> > > > kmalloc interfaces, to provide additional hinting for better bounds
> > > > checking, assisting CONFIG_FORTIFY_SOURCE and other compiler
> > > > optimizations.
> > >
> > > x86_64 allmodconfig:
> >
> > What compiler and version?
> >
> > >
> > > In file included from ./arch/x86/include/asm/preempt.h:7,
> > >                  from ./include/linux/preempt.h:78,
> > >                  from ./include/linux/spinlock.h:55,
> > >                  from ./include/linux/mmzone.h:8,
> > >                  from ./include/linux/gfp.h:6,
> > >                  from ./include/linux/mm.h:10,
> > >                  from ./include/linux/mman.h:5,
> > >                  from lib/test_kasan_module.c:10:
> > > In function 'check_copy_size',
> > >     inlined from 'copy_user_test' at ./include/linux/uaccess.h:191:6:
> > > ./include/linux/thread_info.h:213:4: error: call to '__bad_copy_to' declared with attribute error: copy destination size is too small
> > >   213 |    __bad_copy_to();
> > >       |    ^~~~~~~~~~~~~~~
> > > In function 'check_copy_size',
> > >     inlined from 'copy_user_test' at ./include/linux/uaccess.h:199:6:
> > > ./include/linux/thread_info.h:211:4: error: call to '__bad_copy_from' declared with attribute error: copy source size is too small
> > >   211 |    __bad_copy_from();
> > >       |    ^~~~~~~~~~~~~~~~~
> > > make[1]: *** [lib/test_kasan_module.o] Error 1
> > > make: *** [lib] Error 2
> >
> > Hah, yes, it caught an intentionally bad copy. This may bypass the
> > check, as I've had to do in LKDTM before. I will test...
> >
> > diff --git a/lib/test_kasan_module.c b/lib/test_kasan_module.c
> > index 7ebf433edef3..9fb2fb2937da 100644
> > --- a/lib/test_kasan_module.c
> > +++ b/lib/test_kasan_module.c
> > @@ -19,7 +19,12 @@ static noinline void __init copy_user_test(void)
> >  {
> >         char *kmem;
> >         char __user *usermem;
> > -       size_t size = 128 - KASAN_GRANULE_SIZE;
> > +       /*
> > +        * This is marked volatile to avoid __alloc_size()
> > +        * noticing the intentionally out-of-bounds copys
> > +        * being done on the allocation.
> > +        */
> > +       volatile size_t size = 128 - KASAN_GRANULE_SIZE;
> 
> Maybe OPTIMIZER_HIDE_VAR()? The normal version of that abuses an empty
> asm statement to hide the value from the compiler.

Oh! I hadn't seen that before. Is that better than volatile in this
case?

-- 
Kees Cook