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

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Date:	Mon, 14 Dec 2015 11:18:23 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <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>,
	Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH 1/1] Fix int1 recursion when no perf_bp_event is registeredy

On 12/14/15, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Dec 14, 2015 at 9:52 AM, Jeff Merkey <linux.mdb@...il.com> wrote:
>> On 12/14/15, Ingo Molnar <mingo@...nel.org> wrote:
>>>
>>> * Jeff Merkey <linux.mdb@...il.com> wrote:
>>>
>>>> On 12/14/15, Ingo Molnar <mingo@...nel.org> wrote:
>>>> >
>>>> > A: Because it messes up the order in which people normally read text.
>>>> > Q: Why is top-posting such a bad thing?
>>>> > A: Top-posting.
>>>> > Q: What is the most annoying thing in e-mail?
>>>> >
>>>> > * Jeff Merkey <linux.mdb@...il.com> wrote:
>>>> >
>>>> >> I trigger it by writing to the dr7 and dr1, 2, 3 or four register
>>>> >> and
>>>> >> set an
>>>> >> execute breakpoint without going through arch_install_hw_breakpoint.
>>>> >> When
>>>> >> the breakpoint fires, the system crashes and hangs on the processor
>>>> >> stuck in
>>>> >> an endless loop inside the int1 handler in hw_breakpoint.c --
>>>> >
>>>> > What is still not clear to me, can you trigger the hang not via some
>>>> > special
>>>> >
>>>> > kernel driver that goes outside regular APIs and messes with the
>>>> > state
>>>> > of the
>>>> > debug registers, but via the proper access methods, i.e. various
>>>> > user-space
>>>> > ABIs?
>>>>
>>>> Any process that can get access to the debug registers can trigger this
>>>> condition. [...]
>>>
>>> A process on an unmodified Linux kernel can only modify debug registers
>>> via
>>> the
>>> proper APIs:
>>>
>>>> [...]  As it stands, if restricted to the established API in
>>>> hw_breakpoint.c
>>>> this bug should not occur unless someone triggers an errant breakpoint.
>>>> [...]
>>>
>>> So am I interpreting your report correctly:
>>>
>>>   "If the Linux kernel is modified to change debug registers without
>>> using
>>> the
>>>    proper APIs (such as loading a module that changes hardware registers
>>> in
>>> a raw
>>>    fashion), things may break and a difficult to debug hang may occur."
>>>
>>> right?
>>>
>>> This key piece of information should have been part of the original
>>> report.
>>>
>>> So I'm wondering, why does your module modify debug registers in a raw
>>> fashion?
>>> Why doesn't it use the proper APIs?
>>>
>>> Thanks,
>>>
>>>       Ingo
>>>
>>
>> Hi Ingo,
>>
>> This will be a lengthy reply to properly explain this to you.  First
>> some fundamental assumptions to clear up.
>>
>> 1.  The MDB Debugger Module does not cause this problem.  This is an
>> existing bug in the kernel in an exception code path.
>
>> 8.  If any process or module sets a breakpoint outside of linux
>> breakpoint API in this code path the system will crash.  Its A BUG,
>> and it's been in linux 13 years.  I am certain people have seen it
>> while running perf stuff but since it provides no diagnostic info,
>> someone would just reset the system.
>
> Putting "BUG" in caps doesn't make it so.  What's wrong with it?
>
>> 9.  This breakpoint  API needs to be rewritten to be global breakpoint
>> aware, have an on/off switch so when a debugger enters an int1
>> exception, breakpoints are globally disabled (a requirement), among
>> other things.
>
> A "requirement" for what?
>
>>
>> The patch simply fixes the bug in the int handler that will cause a
>> lockup.  The perf event system, kgdb, kdb, and any one of a number of
>> programs can trigger this bug, and probably have.  People would blame
>> the debugger when its a bug in the int handler.
>
> You ignored feedback from me and from tglx, and you still haven't
> explained why this is a bug in the first place.


It crashes the system -- crash = bug -- right?

Maybe the code could
> degrade more gracefully if you use it wrong, but the int1 handler and
> the rest of the kernel are very much aware of each other, and the int1
> handler's failure to do what something that isn't in the kernel wants
> it to do isn't a bug.
>
> If you submit a clean patch to improve robustness of the handler and
> if the new code is at least as clean as the old code, that might be a
> different story.
>
> --Andy
>
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