linux-kernel - Re: [PATCH v3 28/46] kmsan: entry: handle register passing from uninstrumented code

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Message-ID: <87a6c6y7mg.ffs@tglx>
Date:   Wed, 27 Apr 2022 15:32:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexander Potapenko <glider@...gle.com>, glider@...gle.com
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 28/46] kmsan: entry: handle register passing from
 uninstrumented code

On Tue, Apr 26 2022 at 18:42, Alexander Potapenko wrote:

Can you please use 'entry:' as prefix. Slapping kmsan in front of
everything does not really make sense.

> Replace instrumentation_begin()	with instrumentation_begin_with_regs()
> to let KMSAN handle the non-instrumented code and unpoison pt_regs
> passed from the instrumented part.

That should be:

     from the non-instrumented part
or
     passed to the instrumented part

right?

> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -23,7 +23,7 @@ static __always_inline void __enter_from_user_mode(struct pt_regs *regs)
>  	CT_WARN_ON(ct_state() != CONTEXT_USER);
>  	user_exit_irqoff();
>  
> -	instrumentation_begin();
> +	instrumentation_begin_with_regs(regs);

I can see what you are trying to do, but this will end up doing the same
thing over and over. Let's just look at a syscall.

__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
{
        ...
	nr = syscall_enter_from_user_mode(regs, nr)

  		__enter_from_user_mode(regs)
              		.....
			instrumentation_begin_with_regs(regs);
			....

                instrumentation_begin_with_regs(regs);
                ....                     

	instrumentation_begin_with_regs(regs);

	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
		/* Invalid system call, but still a system call. */
		regs->ax = __x64_sys_ni_syscall(regs);
	}

	instrumentation_end();

        syscall_exit_to_user_mode(regs);
		instrumentation_begin_with_regs(regs);
  		__syscall_exit_to_user_mode_work(regs);
  	instrumentation_end();
  	__exit_to_user_mode();

That means you memset state four times and unpoison regs four times. I'm
not sure whether that's desired.

instrumentation_begin()/end() are not really suitable IMO. They were
added to allow objtool to validate that nothing escapes into
instrumentable code unless annotated accordingly.

Thanks,

        tglx