Re: [PATCH v3 3/5] treewide: use get_random_u32() when possible

Jason A. Donenfeld
2022-10-06T17:25:13Z

Hi Christophe,

On Thu, Oct 6, 2022 at 11:21 AM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
> Le 06/10/2022 à 18:53, Jason A. Donenfeld a écrit :
> > The prandom_u32() function has been a deprecated inline wrapper around
> > get_random_u32() for several releases now, and compiles down to the
> > exact same code. Replace the deprecated wrapper with a direct call to
> > the real function. The same also applies to get_random_int(), which is
> > just a wrapper around get_random_u32().
> >
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Acked-by: Toke Høiland-Jørgensen <toke@toke.dk> # for sch_cake
> > Acked-by: Chuck Lever <chuck.lever@oracle.com> # for nfsd
> > Reviewed-by: Jan Kara <jack@suse.cz> # for ext4
> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> > ---
>
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 0fbda89cd1bb..9c4c15afbbe8 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -2308,6 +2308,6 @@ void notrace __ppc64_runlatch_off(void)
> > unsigned long arch_align_stack(unsigned long sp)
> > {
> > if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
> > - sp -= get_random_int() & ~PAGE_MASK;
> > + sp -= get_random_u32() & ~PAGE_MASK;
> > return sp & ~0xf;
>
> Isn't that a candidate for prandom_u32_max() ?
>
> Note that sp is deemed to be 16 bytes aligned at all time.

Yes, probably. It seemed non-trivial to think about, so I didn't. But
let's see here... maybe it's not too bad:

If PAGE_MASK is always ~(PAGE_SIZE-1), then ~PAGE_MASK is
(PAGE_SIZE-1), so prandom_u32_max(PAGE_SIZE) should yield the same
thing? Is that accurate? And holds across platforms (this comes up a
few places)? If so, I'll do that for a v4.

Jason