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From: Kees Cook <keescook@chromium.org>
To: Will Deacon <will@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Elena Reshetova <elena.reshetova@intel.com>,
	x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Potapenko <glider@google.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Jann Horn <jannh@google.com>,
	"Perla, Enrico" <enrico.perla@intel.com>,
	kernel-hardening@lists.openwall.com,
	linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support
Date: Mon, 20 Apr 2020 15:34:57 -0700
Message-ID: <202004201529.BB787BB@keescook> (raw)
In-Reply-To: <20200420205458.GC29998@willie-the-truck>

On Mon, Apr 20, 2020 at 09:54:58PM +0100, Will Deacon wrote:
> On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote:
> > +	/*
> > +	 * Since the compiler chooses a 4 bit alignment for the stack,
> > +	 * let's save one additional bit (9 total), which gets us up
> > +	 * near 5 bits of entropy.
> > +	 */
> > +	choose_random_kstack_offset(get_random_int() & 0x1FF);
> 
> Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff
> is 9 bits, and that is 4+5 but so what?

Er, well, yes. I guess I was just trying to explain why there were 9
bits saved here and to document what I was seeing the compiler actually
doing with the values. (And it serves as a comparison to the x86 comment
which is explaining similar calculations in the face of x86_64 vs ia32.)

Would something like this be better?

/*
 * Since the compiler uses 4 bit alignment for the stack (1 more than
 * x86_64), let's try to match the 5ish-bit entropy seen in x86_64,
 * instead of having needlessly lower entropy. As a result, keep the
 * low 9 bits.
 */

-- 
Kees Cook

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-24 20:32 [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-24 20:32 ` [PATCH v2 1/5] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2020-03-24 22:06   ` Peter Zijlstra
2020-03-24 20:32 ` [PATCH v2 2/5] init_on_alloc: Unpessimize default-on builds Kees Cook
2020-03-26 15:48   ` Alexander Potapenko
2020-03-24 20:32 ` [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-30 11:25   ` Mark Rutland
2020-03-30 18:18     ` Kees Cook
2020-03-30 18:27     ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 4/5] x86/entry: Enable random_kstack_offset support Kees Cook
2020-03-28 22:26   ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 5/5] arm64: entry: " Kees Cook
2020-03-25 13:21   ` Mark Rutland
2020-03-25 20:22     ` Kees Cook
2020-03-26 11:15       ` Mark Rutland
2020-03-26 16:31         ` Kees Cook
2020-03-30 11:26           ` Mark Rutland
2020-04-20 20:54   ` Will Deacon
2020-04-20 22:34     ` Kees Cook [this message]
2020-04-21  7:02       ` Will Deacon
2020-03-24 21:28 ` [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Jann Horn
2020-03-24 23:07   ` Kees Cook
2020-03-25 12:15     ` Reshetova, Elena
2020-03-25 20:27       ` Kees Cook
2020-03-25 23:20         ` Jann Horn
2020-03-26 17:18           ` Kees Cook

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