linux-kernel - Re: [PATCH 1/4] ARM: Disable jump-label on PREEMPT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241211132607.z1zDTk89@linutronix.de>
Date: Wed, 11 Dec 2024 14:26:07 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org,
	linux-rt-devel@...ts.linux.dev, Ard Biesheuvel <ardb@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Matthew Wilcox <willy@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <linux@...linux.org.uk>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/4] ARM: Disable jump-label on PREEMPT_RT

On 2024-12-10 17:05:53 [+0100], Arnd Bergmann wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> jump-labels are used to efficiently switch between two possible code
> paths. To achieve this, stop_machine() is used to keep the CPU in a
> known state while the opcode is modified. The usage of stop_machine()
> here leads to large latency spikes which can be observed on PREEMPT_RT.
> 
> Jump labels may change the target during runtime and are not restricted
> to debug or "configuration/ setup" part of a PREEMPT_RT system where
> high latencies could be defined as acceptable.
> 
> On 64-bit Arm, it is possible to use jump labels without the
> stop_machine() call, which architecturally provides a way to atomically
> change one 32-bit instruction word while keeping maintaining consistency,
> but this is not generally the case on 32-bit, in particular in thumb2
> mode.
> 
> Disable jump-label support on a PREEMPT_RT system when SMP is enabled.
> 
> [bigeasy: Patch description.]
> [arnd: add !SMP case, extend changelog]
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Link: https://lkml.kernel.org/r/20220613182447.112191-2-bigeasy@linutronix.de
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> I slightly changed the patch from the version currently in linux-rt.git
> to leave jump labels enabled on single-CPU kernels that are still
> fairly common on 32-bit arm.

So HOTPLUG_CPU depends on SMP and without SMP there no HOTPLUG_CPU so
the patch function will be invoked directly. So we patch and flush.
Well, okay.

> If there are no additional concerns about this version, I will
> forward it to Russell's patch system

I'm fine with it, thank you.

Sebastian