linux-kernel - Re: [PATCH 1/2] x86/hw_breakpoint: Prevent data breakpoints on __per_cpu

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Date:   Thu, 4 Feb 2021 16:11:54 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 1/2] x86/hw_breakpoint: Prevent data breakpoints on __per_cpu_offset

On Thu, Feb 4, 2021 at 6:26 AM Lai Jiangshan <jiangshanlai@...il.com> wrote:
>
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>
> When FSGSBASE is enabled, paranoid_entry() fetches the per-CPU
> GSBASE value via __per_cpu_offset or pcpu_unit_offsets.
>
> When data breakpoint is set on __per_cpu_offset[cpu] (read-write
> operation), the specific cpu will be stuck in the infinite #DB loop.
> RCU will try to send NMI to the specific cpu, but it is not working
> either since NMI also relies on paranoid_entry().

Should we consider having a .percpu..noinstr section and having
objtool enforce this?