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

[Jann Horn]

On Wed, Oct 6, 2021 at 5:06 AM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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.