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

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Message-ID: <CALCETrVPXCEcNR08+7Essom9UqaFyMzTAj_XfbFD5NJT3PAoBQ@mail.gmail.com>
Date:	Wed, 16 Dec 2015 16:32:03 -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 Wed, Dec 16, 2015 at 4:31 PM, Jeff Merkey <linux.mdb@...il.com> wrote:
> On 12/16/15, Andy Lutomirski <luto@...capital.net> wrote:
>> 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
>>
>
> Hi Andy,
>
> Could be, but I am getting a double fault message with an error code
> of 0 that then scrolls off the screen when the triple fault hits.  It
> flashes too quickly to get the function address -- wish I had a logic
> analyzer with an inverse assembler -- would already be there.    A
> usermode RSP would I assume clear TRAP flag and that does not explain
> why it works if I set a breakpoint right above the instruction then
> step over it, which I can without the triple fault.
>
> Easy to reproduce, download the mdb debugger for 4.3.3 and apply it to
> 4.4-rc5, modprobe mdb, echo a > /proc/sysrq_trigger, u
> int_ret_from_syscall (scroll til you get to the swapgs then rex64
> sysret, set a hardware breakpoint  at that address , i.e. b
> ffffffff81673ae1 (or whatever address the swapgs instruction is at),
> then step through with t a few times (should just return after rex64
> sysret since it returns to user space).  The set a breakpoint at the
> rex64 sysret instruction, b <address>, let it break at the
> instruction, then hit g for go and watch the fireworks -- it will try
> to print a double fault message then reboot.
>
> I handle the whole user RSP thing, I just return if I see regs set to
> user space.  This looks like some sort of problem in the exception
> handlers.

It's kernel regs but user RSP.

--Andy
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