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

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Date:   Fri, 06 May 2022 18:14:36 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexander Potapenko <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 <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 28/46] kmsan: entry: handle register passing from
 uninstrumented code

On Fri, May 06 2022 at 16:52, Alexander Potapenko wrote:
> On Thu, May 5, 2022 at 11:56 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> @@ -452,6 +455,7 @@ irqentry_state_t noinstr irqentry_nmi_en
>>         rcu_nmi_enter();
>>
>>         instrumentation_begin();
>> +       unpoison(regs);
>>         trace_hardirqs_off_finish();
>>         ftrace_nmi_enter();
>>         instrumentation_end();
>>
>> As I said: 4 places :)
>
> These four instances still do not look sufficient.
> Right now I am seeing e.g. reports with the following stack trace:
>
> BUG: KMSAN: uninit-value in irqtime_account_process_tick+0x255/0x580
> kernel/sched/cputime.c:382
>  irqtime_account_process_tick+0x255/0x580 kernel/sched/cputime.c:382
>  account_process_tick+0x98/0x450 kernel/sched/cputime.c:476
>  update_process_times+0xe4/0x3e0 kernel/time/timer.c:1786
>  tick_sched_handle kernel/time/tick-sched.c:243
>  tick_sched_timer+0x83e/0x9e0 kernel/time/tick-sched.c:1473
>  __run_hrtimer+0x518/0xe50 kernel/time/hrtimer.c:1685
>  __hrtimer_run_queues kernel/time/hrtimer.c:1749
>  hrtimer_interrupt+0x838/0x15a0 kernel/time/hrtimer.c:1811
>  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1086
>  __sysvec_apic_timer_interrupt+0x1ae/0x680 arch/x86/kernel/apic/apic.c:1103
>  sysvec_apic_timer_interrupt+0x95/0xc0 arch/x86/kernel/apic/apic.c:1097
> ...
> (uninit creation stack trace is irrelevant here, because it is some
> random value from the stack)
>
> sysvec_apic_timer_interrupt() receives struct pt_regs from
> uninstrumented code, so regs can be partially uninitialized.
> They are not passed down the call stack directly, but are instead
> saved by set_irq_regs() in sysvec_apic_timer_interrupt() and loaded by
> get_irq_regs() in tick_sched_timer().

sysvec_apic_timer_interrupt() invokes irqentry_enter() _before_
set_irq_regs() and irqentry_enter() unpoisons @reg.

Confused...