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

[Jason A. Donenfeld]

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?

Jason