linux-kernel - Re: [PATCH 08/15] x86/alternatives: Teach text_poke

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Message-ID: <20190611112254.576226fe@gandalf.local.home>
Date:   Tue, 11 Jun 2019 11:22:54 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate
 instructions

On Tue, 11 Jun 2019 10:03:07 +0200
Peter Zijlstra <peterz@...radead.org> wrote:


> So what happens is that arch_prepare_optimized_kprobe() <-
> copy_optimized_instructions() copies however much of the instruction
> stream is required such that we can overwrite the instruction at @addr
> with a 5 byte jump.
> 
> arch_optimize_kprobe() then does the text_poke_bp() that replaces the
> instruction @addr with int3, copies the rel jump address and overwrites
> the int3 with jmp.
> 
> And I'm thinking the problem is with something like:
> 
> @addr: nop nop nop nop nop

What would work would be to:

	add breakpoint to first opcode.

	call synchronize_tasks();

	/* All tasks now hitting breakpoint and jumping over affected
	code */

	update the rest of the instructions.

	replace breakpoint with jmp.

One caveat is that the replaced instructions must not be a call
function. As if the call function calls schedule then it will
circumvent the synchronize_tasks(). It would be OK if that call is the
last of the instructions. But I doubt we modify anything more then a
call size anyway, so this should still work for all current instances.

-- Steve

> 
> We copy out the nops into the trampoline, overwrite the first nop with
> an INT3, overwrite the remaining nops with the rel addr, but oops,
> another CPU can still be executing one of those NOPs, right?
> 
> I'm thinking we could fix this by first writing INT3 into all relevant
> instructions, which is going to be messy, given the current code base.