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Date: Thu, 6 Oct 2022 17:36:48 -0600
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"patches@...ts.linux.dev" <patches@...ts.linux.dev>,
Andreas Noever <andreas.noever@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Borislav Petkov <bp@...en8.de>,
Catalin Marinas <catalin.marinas@....com>,
Christoph Böhmwalder
<christoph.boehmwalder@...bit.com>, Christoph Hellwig <hch@....de>,
Daniel Borkmann <daniel@...earbox.net>,
Dave Airlie <airlied@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Florian Westphal <fw@...len.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"H . Peter Anvin" <hpa@...or.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Helge Deller <deller@....de>,
Herbert Xu <herbert@...dor.apana.org.au>,
Huacai Chen <chenhuacai@...nel.org>,
Hugh Dickins <hughd@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
"James E . J . Bottomley" <jejb@...ux.ibm.com>,
Jan Kara <jack@...e.com>, Jason Gunthorpe <jgg@...pe.ca>,
Jens Axboe <axboe@...nel.dk>,
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Jonathan Corbet <corbet@....net>,
Jozsef Kadlecsik <kadlec@...filter.org>,
KP Singh <kpsingh@...nel.org>,
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Toke Høiland-Jørgensen <toke@...e.dk>,
Chuck Lever <chuck.lever@...cle.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3 3/5] treewide: use get_random_u32() when possible
On 10/6/22, Christophe Leroy <christophe.leroy@...roup.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.leroy@...roup.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@...omium.org>
>>>>> Acked-by: Toke Høiland-Jørgensen <toke@...e.dk> # for sch_cake
>>>>> Acked-by: Chuck Lever <chuck.lever@...cle.com> # for nfsd
>>>>> Reviewed-by: Jan Kara <jack@...e.cz> # for ext4
>>>>> Signed-off-by: Jason A. Donenfeld <Jason@...c4.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?
Jason
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