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Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190428154345.789635b3@oasis.local.home>
Date:   Sun, 28 Apr 2019 13:56:31 -0700
Cc:     Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Andy Lutomirski <luto@kernel.org>,
        Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org, linux-kselftest@vger.kernel.org
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References: <20190427100639.15074-1-nstange@suse.de> <20190427100639.15074-2-nstange@suse.de> <F6268634-96D3-4267-B739-6D375FAB26A8@amacapital.net> <20190428135143.09d35bb6@oasis.local.home> <D5735AF7-D603-4B20-A3EE-B519044D1E5D@amacapital.net> <20190428154345.789635b3@oasis.local.home>
To:     Steven Rostedt <rostedt@goodmis.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 28, 2019, at 12:43 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
>=20
> On Sun, 28 Apr 2019 11:08:34 -0700
> Andy Lutomirski <luto@amacapital.net> wrote:
>=20
>>>=20
>>> Perhaps adding another slot into pt_regs that gets used by int3 to
>>> store a slot to emulate a call on return?
>>>=20
>>>=20
>>=20
>> That=E2=80=99s not totally nuts, although finding pt_regs isn=E2=80=99t e=
ntirely trivial.
>=20
> I meant on the int3 handler (which stores the pt_regs).

But that=E2=80=99s below the stub=E2=80=99s RSP, so it=E2=80=99s toast if an=
other interrupt happens. Or am I misunderstanding you?

>=20
>>=20
>> I still think I prefer an approach where we just emulate the call directl=
y.
>=20
> 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=E2=80=99s old proposal of unconditionally shifting the #BP fram=
e 8 bytes better. It will be interesting when kernel shadow stacks are throw=
n in the mix, but that=E2=80=99s a problem for another day.