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

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Date:	Mon, 14 Dec 2015 01:13:10 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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, 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?
>
> Thanks,
>
> 	Ingo
>

Any process that can get access to the debug registers can trigger
this condition.  As it stands, if restricted to the established API in
hw_breakpoint.c this bug should not occur unless someone triggers an
errant breakpoint.  That being said, there is a severe bug in the code
path if for some reason an application triggers a breakpoint exception
and no event has been registered, the system will crash with no logged
output of any kind indicating why it happened.  It's not severe enough
for a panic but does need to be handled gracefully just as exception
handling 101.

Sorry about the top posting.  I forget sometimes.

Jeff
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