linux-kernel - Re: [PATCH 1/1] Fix int1 recursion when no perf_bp

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Date:	Thu, 10 Dec 2015 19:55:35 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jeff Merkey <linux.mdb@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/1] Fix int1 recursion when no perf_bp_event is
 registered

Jeff,

On Thu, 10 Dec 2015, Jeff Merkey wrote:

> If an int1 hardware breakpoint exception is triggered, but no perf bp
> pevent block was registered from arch_install_hw_breakpoint, the
> system will hard hang with the CPU stuck constantly re-interrupting at
> the same execution address because the resume flag never gets set, and
> the NOTIFY_DONE state prevents other int1 handlers, including the
> default handler in do_debug, from running to handle the condition.
> Can be reproduced by writing a program that sets an execute breakpoint
> at schedule() without calling arch_install_hw_breakpoint.
> 
> The proposed fix checks the dr7 register and sets the resume flag in
> pt->regs if it determines an executed breakpoint was triggered just in
> case the check lower down fails.  I have seen this bug and its a bug.

> Signed-off-by:  jeffmerkey@...il.com
> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> index 50a3fad..6effcae 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -475,6 +475,14 @@ static int hw_breakpoint_handler(struct die_args *args)
>  	for (i = 0; i < HBP_NUM; ++i) {
>  		if (likely(!(dr6 & (DR_TRAP0 << i))))
>  			continue;
> +		/*
> +		 * Set up resume flag to avoid breakpoint recursion when
> +		 * returning back to origin in the event an int1
> +		 * exception is triggered and no event handler
> +		 * is present.
> +		 */
> +		if ((dr7 & (3 << ((i * 4) + 16))) == 0)

We have proper defines for all of this. See __encode_dr7().

> +			args->regs->flags |= X86_EFLAGS_RF;

If there is a break point installed, then we do the same thing after
calling perf_bp_event() again.

Thanks,

	tglx
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