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:   Sun, 20 Jun 2021 13:23:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        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 20:13, Andy Lutomirski wrote:
> 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.

Bah.

> None of this is NMI-specific. #DB itself has the same problem.

Oh well.

> 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.

That's ugly and TBH in some sense is a breakpoint on the thread stack a
violation of noinstr. I rather see them prevented completely, but yes
that would have to be expanded to pretty much any variable which is
touched in noinstr sections. What a mess.

Thanks,

        tglx