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

Christophe Leroy
2022-10-06T17:43:11Z

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@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.
>>
>
> 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;