linux-kernel - Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3

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Date:   Sun, 28 Apr 2019 13:56:31 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andy Lutomirski <luto@...nel.org>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member


> On Apr 28, 2019, at 12:43 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Sun, 28 Apr 2019 11:08:34 -0700
> Andy Lutomirski <luto@...capital.net> wrote:
> 
>>> 
>>> Perhaps adding another slot into pt_regs that gets used by int3 to
>>> store a slot to emulate a call on return?
>>> 
>>> 
>> 
>> That’s not totally nuts, although finding pt_regs isn’t entirely trivial.
> 
> I meant on the int3 handler (which stores the pt_regs).

But that’s below the stub’s RSP, so it’s toast if another interrupt happens. Or am I misunderstanding you?

> 
>> 
>> I still think I prefer an approach where we just emulate the call directly.
> 
> Then, on the return of int3, if there's anything in that slot, then we
> could possibly shift the exception handler frame (that was added by the
> hardware), insert the slot data into the top of the stack, and then
> call iret (which the int3 handler, would add the return ip to be the
> function being called), which would in essence emulate the call directly.

Oh, I get it.

I liked Josh’s old proposal of unconditionally shifting the #BP frame 8 bytes better. It will be interesting when kernel shadow stacks are thrown in the mix, but that’s a problem for another day.