Received-SPF: pass (google.com: domain of linux-kernel+bounces-96298-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.11.0-alpha0-251-g8332da0bf6-fm-20240305.001-g8332da0b
Precedence: bulk
MIME-Version: 1.0
Message-Id: <db9bc2dc-6a66-4248-b8ab-abc2723af053@app.fastmail.com>
In-Reply-To: <202403071112.01B4579@keescook>
References: <20240305221824.3300322-1-jeremy.linton@arm.com>
 <20240305221824.3300322-2-jeremy.linton@arm.com>
 <202403051526.0BE26F99E@keescook>
 <34351804-ad1d-498f-932a-c1844b78589f@app.fastmail.com>
 <38f9541b-dd88-4d49-af3b-bc7880a4e2f4@arm.com>
 <f1dd15ce-69af-4315-8d7c-b7a480e541aa@app.fastmail.com>
 <202403071112.01B4579@keescook>
Date: Thu, 07 Mar 2024 23:02:44 +0100
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Kees Cook" <keescook@chromium.org>
Cc: "Jeremy Linton" <jeremy.linton@arm.com>,
 linux-arm-kernel@lists.infradead.org,
 "Catalin Marinas" <catalin.marinas@arm.com>, "Will Deacon" <will@kernel.org>,
 "Jason A . Donenfeld" <Jason@zx2c4.com>,
 "Gustavo A. R. Silva" <gustavoars@kernel.org>,
 "Mark Rutland" <mark.rutland@arm.com>,
 "Steven Rostedt" <rostedt@goodmis.org>, "Mark Brown" <broonie@kernel.org>,
 "Guo Hui" <guohui@uniontech.com>, Manoj.Iyer@arm.com,
 linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
 "James Yang" <james.yang@arm.com>, "Shiyou Huang" <shiyou.huang@arm.com>
Subject: Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization
Content-Type: text/plain

On Thu, Mar 7, 2024, at 20:15, Kees Cook wrote:
> On Thu, Mar 07, 2024 at 12:10:34PM +0100, Arnd Bergmann wrote:
>> There is not even any attempt to use the most random bits of
>> the cycle counter, as both the high 22 to 24 bits get masked
>> out (to keep the wasted stack space small) and the low 3 or 4
>> bits get ignored because of stack alignment. If there was
>> any desire to make it more random, a trivial improvement
>> would be:
>> 
>> +++ b/include/linux/randomize_kstack.h
>> @@ -80,7 +80,7 @@ DECLARE_PER_CPU(u32, kstack_offset);
>>         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
>>                                 &randomize_kstack_offset)) {            \
>>                 u32 offset = raw_cpu_read(kstack_offset);               \
>> -               offset ^= (rand);                                       \
>> +               offset = ror32(offset, 5) & (rand);                     \
>
> Shouldn't this stay ^ instead of & ?

Yes, my mistake.

>>                 raw_cpu_write(kstack_offset, offset);                   \
>>         }                                                               \
>>  } while (0)
>
> But yeah, we should likely make this change regardless.

Ok, do you want to send a patch, or should I?

     Arnd