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

From: Christophe Leroy
Date: Thu Oct 06 2022 - 13:43:15 EST

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;