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

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Date:   Mon, 26 Oct 2020 17:05:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kyle Huey <me@...ehuey.com>
Cc:     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>,
        Andy Lutomirski <luto@...nel.org>,
        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 Mon, Oct 26, 2020 at 04:55:21PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 26, 2020 at 07:33:08AM -0700, Kyle Huey wrote:
> > After resuming a ptracee with PTRACE_SINGLESTEP, in the following
> > ptrace stop retrieving the dr6 value for the tracee gets a value that
> > does not include DR_STEP (it is in fact always DR6_RESERVED). I
> > bisected this to the 13cb73490f475f8e7669f9288be0bcfa85399b1f merge. I
> > did not bisect further.
> > 
> > I don't see any handling to ever set DR_STEP in virtual_dr6, so I
> > think this code is just broken.
> > 
> > Sorry for not testing this when I was CCd on the original patch series :)
> 
> Urgh, now I have to try and remember how all that worked again ...
> 
> I suspect it's either one (or both) of the last two:
> 
>   f4956cf83ed1 ("x86/debug: Support negative polarity DR6 bits")
>   d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")
> 
> 
> Just to clarify, the sequence is something like:
> 
>  - tracer: ptrace(PTRACE_SINGLESTEP)
>  - tracee: #DB, DR6 contains DR_STEP
>  - tracer: ptrace_get_debugreg(6)
> 
> ?
> 
> You're right that that would be broken, let me try and figure out what
> the best way would be 'fix' that.
> 
> Also, can you confirm that pthread_set_debugreg(6) should not do
> anything useful?


Does something like this make sense?


diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 3c70fb34028b..0e7641ac19a8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -799,6 +799,13 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
 	 */
 	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;
+
 	/*
 	 * The SDM says "The processor clears the BTF flag when it
 	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep