linux-kernel - Re: [PATCH] remove AND operation in choose_random_kstack

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Message-Id: <dbca95d9-4818-4624-b3c2-4b51284fe83b@app.fastmail.com>
Date: Tue, 18 Jun 2024 13:14:58 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Mark Rutland" <mark.rutland@....com>
Cc: "Kees Cook" <kees@...nel.org>, "Yuntao Liu" <liuyuntao12@...wei.com>,
 x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
 linux-hardening@...r.kernel.org, "Catalin Marinas" <catalin.marinas@....com>,
 "Will Deacon" <will@...nel.org>, "Heiko Carstens" <hca@...ux.ibm.com>,
 gor@...ux.ibm.com, "Alexander Gordeev" <agordeev@...ux.ibm.com>,
 "Christian Borntraeger" <borntraeger@...ux.ibm.com>,
 "Sven Schnelle" <svens@...ux.ibm.com>,
 "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "Leonardo Bras" <leobras@...hat.com>, "Mark Brown" <broonie@...nel.org>,
 imbrenda@...ux.ibm.com, pawan.kumar.gupta@...ux.intel.com
Subject: Re: [PATCH] remove AND operation in choose_random_kstack_offset()

On Tue, Jun 18, 2024, at 12:45, Mark Rutland wrote:
> On Mon, Jun 17, 2024 at 10:33:08PM +0200, Arnd Bergmann wrote:
>> On Mon, Jun 17, 2024, at 20:22, Kees Cook wrote:
>> > On Mon, Jun 17, 2024 at 04:52:15PM +0100, Mark Rutland wrote:

> Sorry, to be clear, I'm happy for this to change, so long as:
>
> * The commit message explains why that's safe.
>
>   IIUC this goes from 511 to 1023 bytes on arm64, which is ~3% of the
>   stack, so maybe that is ok. It'd be nice to see any rationale/analysis
>   beyond "the offset would be bitwise ANDed with 0x3FF".

Absolutely agreed, and the commit message should also clarify that
the increase has already happened as an unintended side-effect
of commit 9c573cd31343 ("randomize_kstack: Improve entropy
diffusion").

> * The comments in architecture code referring to the masking get
>   removed/updated along with the masking.

Right.

FWIW, I also wouldn't mind to having a compile-time option
that configures the number of random bits on the stack offset,
but my preference here is to have a reasonable default and
not need a config option.

    Arnd