linux-kernel - Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR

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Message-ID: <20201027192009.GQ2628@hirez.programming.kicks-ass.net>
Date:   Tue, 27 Oct 2020 20:20:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Kyle Huey <me@...ehuey.com>,
        open list <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Robert O'Callahan <rocallahan@...il.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        "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>,
        Joel Fernandes <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no
 longer reported in dr6

On Tue, Oct 27, 2020 at 11:00:52AM -0700, Andy Lutomirski wrote:
> On Tue, Oct 27, 2020 at 1:19 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Mon, Oct 26, 2020 at 04:30:32PM -0700, Andy Lutomirski wrote:
> >
> > > > @@ -935,6 +936,26 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
> > > >         irqentry_enter_from_user_mode(regs);
> > > >         instrumentation_begin();
> > > >
> > > > +       /*
> > > > +        * Clear the virtual DR6 value, ptrace routines will set bits here for
> > > > +        * things we want signals for.
> > > > +        */
> > > > +       current->thread.virtual_dr6 = 0;
> > > > +
> > > > +       /*
> > > > +        * If PTRACE requested SINGLE(BLOCK)STEP, make sure to reflect that in
> > > > +        * the ptrace visible DR6 copy.
> > > > +        */
> > > > +       if (test_thread_flag(TIF_BLOCKSTEP) || test_thread_flag(TIF_SINGLESTEP))
> > > > +               current->thread.virtual_dr6 |= (dr6 & DR_STEP);
> > >
> > > I'm guessing that this would fail a much simpler test, though: have a
> > > program use PUSHF to set TF and then read out DR6 from the SIGTRAP.  I
> > > can whip up such a test if you like.
> >
> > Kyle also mentioned it. The reason I didn't do that is because ptrace()
> > didn't set the TF, so why should it see it in ptrace_get_debugreg(6) ?
> 
> I assume you already figured this out, but my specific concern is with
> the get_si_code(dr6) part -- that's sent directly to the task being
> debugged or debugging itself (and, sadly, to ptrace, and who knows
> what debuggers do).

Right, so for a task doing TF on its own, DR_STEP should remain set in
our on-stack dr6 variable, nothing should consume it.

So the get_si_code(dr6) should be identical. So the only difference is
if DR_STEP is visible in ptrace or not.