linux-kernel - Re: [RFC PATCH 1/4] x86/entry/nmi: Switch to the entry stack before switching to the thread stack

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Date:   Sat, 19 Jun 2021 20:13:15 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Lai Jiangshan" <jiangshanlai@...il.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Cc:     "Steven Rostedt" <rostedt@...dmis.org>,
        "Lai Jiangshan" <laijs@...ux.alibaba.com>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Juergen Gross" <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Arvind Sankar" <nivedita@...m.mit.edu>
Subject: Re: [RFC PATCH 1/4] x86/entry/nmi: Switch to the entry stack before switching to the thread stack



On Sat, Jun 19, 2021, at 3:51 PM, Thomas Gleixner wrote:
> On Tue, Jun 01 2021 at 14:52, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@...ux.alibaba.com>
> >
> > Current kernel has no code to enforce data breakpoint not on the thread
> > stack.  If there is any data breakpoint on the top area of the thread
> > stack, there might be problem.
> 
> And because the kernel does not prevent data breakpoints on the thread
> stack we need to do more complicated things in the already horrible
> entry code instead of just doing the obvious and preventing data
> breakpoints on the thread stack?

Preventing breakpoints on the thread stack is a bit messy: it’s possible for a breakpoint to be set before the address in question is allocated for the thread stack.

None of this is NMI-specific. #DB itself has the same problem.  We could plausibly solve it differently by disarming breakpoints in the entry asm before switching stacks. I’m not sure how much I like that approach.

> 
> Confused.
> 
> Thanks,
> 
>         tglx
>