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

[Christophe Leroy]

Le 06/10/2022 à 19:31, Christophe Leroy a écrit :
> Le 06/10/2022 à 19:24, Jason A. Donenfeld a écrit :
> > Hi Christophe,
> >
> > On Thu, Oct 6, 2022 at 11:21 AM Christophe Leroy
> > <christophe.leroy@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxx> # for sch_cake
> > > > Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx> # for nfsd
> > > > Reviewed-by: Jan Kara <jack@xxxxxxx> # for ext4
> > > > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> > > > ---
> > >
> > > > 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.
>
> On powerpc it is always (from arch/powerpc/include/asm/page.h) :
>
> /*
>   * Subtle: (1 << PAGE_SHIFT) is an int, not an unsigned long. So if we
>   * assign PAGE_MASK to a larger type it gets extended the way we want
>   * (i.e. with 1s in the high bits)
>   */
> #define PAGE_MASK      (~((1 << PAGE_SHIFT) - 1))
>
> #define PAGE_SIZE        (1UL << PAGE_SHIFT)
>
>
> So it would work I guess.

But taking into account that sp must remain 16 bytes aligned, would it 
be better to do something like ?

	sp -= prandom_u32_max(PAGE_SIZE >> 4) << 4;

	return sp;