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

[Jason A. Donenfeld]

On Fri, Oct 07, 2022 at 04:57:24AM +0000, Christophe Leroy wrote:
> 
> 
> Le 07/10/2022 à 01:36, Jason A. Donenfeld a écrit :
> > On 10/6/22, Christophe Leroy <christophe.leroy@xxxxxxxxxx> wrote:
> >>
> >>
> >> 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;
> > 
> > Does this assume that sp is already aligned at the beginning of the
> > function? I'd assume from the function's name that this isn't the
> > case?
> 
> Ah you are right, I overlooked it.

So I think to stay on the safe side, I'm going to go with
`prandom_u32_max(PAGE_SIZE)`. Sound good?

Jason