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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Alexander Potapenko <glider@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
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Subject: Re: [PATCH v3 28/46] kmsan: entry: handle register passing from
 uninstrumented code
In-Reply-To: <CAG_fn=XVchXCcOhFt+rP=vinRhkyrXJSP46cyvcZeHJWaDquGg@mail.gmail.com>
References: <20220426164315.625149-1-glider@google.com>
 <20220426164315.625149-29-glider@google.com> <87a6c6y7mg.ffs@tglx>
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 <87y1zjlhmj.ffs@tglx>
 <CAG_fn=XxAhBEBP2KJvahinbaxLAd1xvqTfRJdAu1Tk5r8=01jw@mail.gmail.com>
 <878rrfiqyr.ffs@tglx>
 <CAG_fn=XVchXCcOhFt+rP=vinRhkyrXJSP46cyvcZeHJWaDquGg@mail.gmail.com>
Date:   Fri, 06 May 2022 18:14:36 +0200
Message-ID: <87k0ayhc43.ffs@tglx>
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On Fri, May 06 2022 at 16:52, Alexander Potapenko wrote:
> On Thu, May 5, 2022 at 11:56 PM Thomas Gleixner <tglx@linutronix.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...