Le 07/10/2022 à 01:36, Jason A. Donenfeld a écrit : > On 10/6/22, Christophe Leroy <christophe.le...@csgroup.eu> 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.le...@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 <keesc...@chromium.org> >>>>>> Acked-by: Toke Høiland-Jørgensen <t...@toke.dk> # for sch_cake >>>>>> Acked-by: Chuck Lever <chuck.le...@oracle.com> # for nfsd >>>>>> Reviewed-by: Jan Kara <j...@suse.cz> # for ext4 >>>>>> Signed-off-by: Jason A. Donenfeld <ja...@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; > > 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. Looking in more details, I see that all architectures that implement it implement it almost the same way. By the way, the comment in arch/um/kernel/process.c is overdated. Most architectures AND the random value with ~PAGE_MASK, x86 and um use %8192. Seems like at the time 2.6.12 was introduced into git, only i386 x86_64 and um had that function. Maybe it is time for a cleanup and a refactoring ? Architectures would just have to provide STACK_ALIGN just like loongarch does today, and we could get a generic arch_align_stack() ? Christophe