Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
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>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
Date: Thu, 29 Nov 2018 09:02:23 -0800	[thread overview]
Message-ID: <0A629D30-ADCF-4159-9443-E5727146F65F@amacapital.net> (raw)
In-Reply-To: <CAHk-=wg5wUXJX5mF+22WXWM_vPESb+p6-z-ZrG7rC1k2O7u=NQ@mail.gmail.com>

> On Nov 29, 2018, at 8:50 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
>> On Thu, Nov 29, 2018 at 8:33 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>> 
>> This seems to work...
>> 
>> +       .if \create_gap == 1
>> +       .rept 6
>> +       pushq 5*8(%rsp)
>> +       .endr
>> +       .endif
>> +
>> -idtentry int3                  do_int3                 has_error_code=0
>> +idtentry int3                  do_int3                 has_error_code=0        create_gap=1
> 
> Ugh. Doesn't this entirely screw up the stack layout, which then
> screws up  task_pt_regs(), which then breaks ptrace and friends?
> 
> ... 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.
> 
> 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.
> 
> So no. Do *not* try to change %rsp on the stack in the bp handler.
> Instead, I'd suggest:
> 
> - just restart the instruction (with the suggested "ptregs->rip --")
> 
> - 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.
> 
> 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 handle the sync_core IPI.

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