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

[Christophe Leroy]

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.

Christophe