linux-kernel - Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

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Message-ID: <nycvar.YFH.7.76.1905022101060.10635@cbobk.fhfr.pm>
Date:   Thu, 2 May 2019 21:28:53 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        the arch/x86 maintainers <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        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>,
        Joerg Roedel <jroedel@...e.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions

On Thu, 2 May 2019, Linus Torvalds wrote:

> I forget: is #BP _only_ for the "int3" instruction? 

Hmm, according to 17.3.2 in vol 3 of SDM (and table 6-1 there), that 
indeed seems to be the case, so we should be fine.

> But if "int3 from kernel space" _only_ happens on actual "int3"
> instructions, then we really could just special-case that case. We'd
> know that %cr3 has been switched, we'd know that we don't need to do
> fsgs switching, we'd know we already have a good stack and percpu data
> etc set up.

That should indeed be doable, under the asumption that noone is doing any 
int3 games before we've switched away from entry trampoline.

I've briefly looked, and seems like we have proper notrace anotations for 
stackleak_erase(), which seems to be the only C (ftrace-able) code that's 
running on a trampoline (off-topic: where does PTI code make sure that we 
actually map this symbol into user pagetables in case we're not doing 
global mapping?).

-- 
Jiri Kosina
SUSE Labs