linux-kernel - Re: 4.4-rc5 Setting hardware breakpoint in int_ret_from_sys

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Message-ID: <CALCETrUDZRAGahZqf76Wh3a-JLbZHUY9UfUtmpzxiZmhfmhTCQ@mail.gmail.com>
Date:	Wed, 16 Dec 2015 16:01:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jeff Merkey <linux.mdb@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: 4.4-rc5 Setting hardware breakpoint in int_ret_from_sys_call
 causes triple fault/reboot

On Dec 16, 2015 3:12 PM, "Jeff Merkey" <linux.mdb@...il.com> wrote:
>
> Setting a hardware breakpoint at the
>
> rex64 sysret
>
> instruction at the end of int_ret_from_sys_call causes the system to
> triple fault
> and reboot when the breakpoint is triggered.  Appears to be related
> the same problem
> as the lockup.
>
> This function can be stepped over and traced through with the TRAP
> FLAG set so long as a hardware breakpoint is set somewhere in the
> function.  Otherwise upon exist the system hard hangs.  If you break
> exactly on that instruction -- reboot.   If you break a few
> instructions before it and single step through the call it works.  If
> you step through the call with no breakpoint the system hard hangs.
> Same behavior as when you try to step from inside an nmi handler.
> Looks related.

You're probably encountering the user mode RSP when SYSRET happens.

--Andy
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