linux-kernel - Re: [PATCH] x86/debug: 'Fix' ptrace dr6 output

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Message-ID: <3ae6318c-b7b5-5174-af6e-c7f7241e9a47@citrix.com>
Date:   Thu, 28 Jan 2021 18:45:38 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>, <x86@...nel.org>,
        <tdevries@...e.de>
CC:     <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] x86/debug: 'Fix' ptrace dr6 output

On 28/01/2021 18:20, Peter Zijlstra wrote:
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -496,9 +496,32 @@ static int hw_breakpoint_handler(struct
>  	dr6_p = (unsigned long *)ERR_PTR(args->err);
>  	dr6 = *dr6_p;
>  
> -	/* If it's a single step, TRAP bits are random */
> -	if (dr6 & DR_STEP)
> +	/*
> +	 * If DR_STEP is set, TRAP bits might also be set, but we must not
> +	 * process them since another exception (without DR_STEP) will follow.

:) How lucky are you feeling?

Data breakpoints will in principle merge with DR_STEP (as will all other
#DB's with trap semantics), other than the many errata about breakpoints
not being recognised correctly.

Under VT-x because there is a still unfixed vmexit microcode bug which
loses all breakpoint information if DR_STEP is set.  %dr6 handling works
fine when #DB isn't intercepted, but then you get to keep the pipeline
livelock vulnerability as a consequence.

Instruction breakpoints on the following instruction will be delivered
as a second #DB, because fault style #DBs are raised at a separate
position in the instruction execution cycle.

~Andrew