linux-kernel - Re: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

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Date:   Wed, 19 Aug 2020 17:47:31 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kyle Huey <me@...ehuey.com>, Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Robert O'Callahan <rocallahan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

On Wed, Aug 19, 2020 at 11:35:34PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 19, 2020 at 12:28:16PM -0700, Kyle Huey wrote:
> 
> > > I'm guess that is not the expected outcome, is that the same failure you
> > > saw?
> > 
> > Yes. Is status also 0x4d00 for you?
> 
> Indeed.
> 
> > The program is expected to complete with no assertions firing.
> 
> When I comment out the break-on-exec test, the break-on-write test
> succeeds.
> 
> When I add a few printk()'s to our #DB handler (6) the program will
> magically work again.

I added some trace_printk()'s and I think the #DB handler is calling
schedule????

exc_debug_user()
  irqentry_exit_to_user_mode()
    exit_to_user_mode_prepare()
      exit_to_user_mode_loop()
        schedule()

So #DB schedules out, then the process scheduls in and tells ptrace to
set the data breakpoint in DR7.  Then the #DB handler schedules back in
and overwrites DR7 with the original value.

What amazes me is that it successfully schedules back to the end of the
#DB handler finish and everything keeps working.

Do we not have assertions in the scheduler to catch this?

-- 
Josh