Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAHk-=wg5wUXJX5mF+22WXWM_vPESb+p6-z-ZrG7rC1k2O7u=NQ@mail.gmail.com>
Date:   Thu, 29 Nov 2018 09:02:23 -0800
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <0A629D30-ADCF-4159-9443-E5727146F65F@amacapital.net>
References: <20181126160217.GR2113@hirez.programming.kicks-ass.net> <20181126171036.chcbmb35ygpxziub@treble> <20181126175624.bruqfbkngbucpvxr@treble> <20181126200801.GW2113@hirez.programming.kicks-ass.net> <20181126212628.4apztfazichxnt7r@treble> <20181127084330.GX2113@hirez.programming.kicks-ass.net> <CALCETrXam8JBtXE3S97Vyb_=+pmjgr23SoZ-Dm5pmYtwZYA+7Q@mail.gmail.com> <20181129094210.GC2131@hirez.programming.kicks-ass.net> <ECF632D9-581B-40B7-9F0A-6B5607A4D0E3@amacapital.net> <20181129143853.GO2131@hirez.programming.kicks-ass.net> <20181129163342.tp5wlfcyiazwwyoh@treble> <CAHk-=wg5wUXJX5mF+22WXWM_vPESb+p6-z-ZrG7rC1k2O7u=NQ@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 29, 2018, at 8:50 AM, Linus Torvalds <torvalds@linux-foundation.org=
> wrote:
>=20
>> On Thu, Nov 29, 2018 at 8:33 AM Josh Poimboeuf <jpoimboe@redhat.com> wrot=
e:
>>=20
>> This seems to work...
>>=20
>> +       .if \create_gap =3D=3D 1
>> +       .rept 6
>> +       pushq 5*8(%rsp)
>> +       .endr
>> +       .endif
>> +
>> -idtentry int3                  do_int3                 has_error_code=3D=
0
>> +idtentry int3                  do_int3                 has_error_code=3D=
0        create_gap=3D1
>=20
> Ugh. Doesn't this entirely screw up the stack layout, which then
> screws up  task_pt_regs(), which then breaks ptrace and friends?
>=20
> ... and you'd only notice it for users that use int3 in user space,
> which now writes random locations on the kernel stack, which is then a
> huge honking security hole.
>=20
> It's possible that I'm confused, but let's not play random games with
> the stack like this. The entry code is sacred, in scary ways.
>=20
> So no. Do *not* try to change %rsp on the stack in the bp handler.
> Instead, I'd suggest:
>=20
> - just restart the instruction (with the suggested "ptregs->rip --")
>=20
> - to avoid any "oh, we're not making progress" issues, just fix the
> instruction yourself to be the right call, by looking it up in the
> "what needs to be fixed" tables.
>=20
> No?

I thought that too.  I think it deadlocks. CPU A does text_poke_bp().  CPU B=
 is waiting for a spinlock with IRQs off.  CPU C holds the spinlock and hits=
 the int3.  The int3 never goes away because CPU A is waiting for CPU B to h=
andle the sync_core IPI.

Or do you think we can avoid the IPI while the int3 is there?=