linux-hardening - Re: [PATCH v3 1/3] randomize_kstack: Maintain kstack

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Message-ID: <19a0da52-5322-40b0-9195-5d191f582bc5@arm.com>
Date: Mon, 5 Jan 2026 10:30:02 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Laight <david.laight.linux@...il.com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Huacai Chen <chenhuacai@...nel.org>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Paul Walmsley <pjw@...nel.org>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...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>,
 Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, Mark Rutland <mark.rutland@....com>,
 "Jason A. Donenfeld" <Jason@...c4.com>, Ard Biesheuvel <ardb@...nel.org>,
 Jeremy Linton <jeremy.linton@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, linux-hardening@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v3 1/3] randomize_kstack: Maintain kstack_offset per task

On 02/01/2026 22:44, David Laight wrote:
> On Fri,  2 Jan 2026 13:11:52 +0000
> Ryan Roberts <ryan.roberts@....com> wrote:
> 
>> kstack_offset was previously maintained per-cpu, but this caused a
>> couple of issues. So let's instead make it per-task.
>>
>> Issue 1: add_random_kstack_offset() and choose_random_kstack_offset()
>> expected and required to be called with interrupts and preemption
>> disabled so that it could manipulate per-cpu state. But arm64, loongarch
>> and risc-v are calling them with interrupts and preemption enabled. I
>> don't _think_ this causes any functional issues, but it's certainly
>> unexpected and could lead to manipulating the wrong cpu's state, which
>> could cause a minor performance degradation due to bouncing the cache
>> lines. By maintaining the state per-task those functions can safely be
>> called in preemptible context.
>>
>> Issue 2: add_random_kstack_offset() is called before executing the
>> syscall and expands the stack using a previously chosen rnadom offset.
>                                                            <>
> 	David

Cheers; will fix in next version.

Thanks,
Ryan