From: Kees Cook <keescook@chromium.org>
To: Arnd Bergmann <arnd@arndb.de>
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
Date: Thu, 7 Mar 2024 11:15:23 -0800 [thread overview]
Message-ID: <202403071112.01B4579@keescook> (raw)
In-Reply-To: <f1dd15ce-69af-4315-8d7c-b7a480e541aa@app.fastmail.com>
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 & ?
> raw_cpu_write(kstack_offset, offset); \
> } \
> } while (0)
But yeah, we should likely make this change regardless.
> My impression is that is is already bordering on becoming
> a "bespoke rng" implementation that Jason was objecting to,
> so the current version is intentionally left weak in order
> to not even give the appearance of being a security relevant
> feature.
I don't think it's bad to make a trivial improvement to entropy diffusion.
--
Kees Cook
next prev parent reply other threads:[~2024-03-07 19:15 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-05 22:18 [PATCH 0/1] Bring kstack randomized perf closer to unrandomized Jeremy Linton
2024-03-05 22:18 ` [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization Jeremy Linton
2024-03-05 23:33 ` Kees Cook
2024-03-06 20:46 ` Arnd Bergmann
2024-03-06 21:54 ` Jeremy Linton
2024-03-07 11:10 ` Arnd Bergmann
2024-03-07 19:10 ` Kees Cook
2024-03-07 21:56 ` Arnd Bergmann
2024-03-07 19:15 ` Kees Cook [this message]
2024-03-07 22:02 ` Arnd Bergmann
2024-03-08 16:49 ` Jeremy Linton
2024-03-08 20:29 ` Arnd Bergmann
2024-03-22 23:40 ` Jeremy Linton
2024-03-23 12:47 ` Arnd Bergmann
2024-03-07 19:05 ` kernel test robot
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