Message-ID: <20211123110944.GB37253@lakrids.cambridge.arm.com>
Date:   Tue, 23 Nov 2021 11:09:44 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     catalin.marinas@....com, dvyukov@...gle.com, peterz@...radead.org,
        quic_qiancai@...cinc.com, valentin.schneider@....com,
        will@...nel.org, woodylin@...gle.com
Subject: Re: [PATCH v2] Reset task stack state in bringup_cpu()

On Tue, Nov 23, 2021 at 11:06:48AM +0000, Mark Rutland wrote:
> On Thu, Nov 18, 2021 at 10:29:27AM +0000, Mark Rutland wrote:
> > To hot unplug a CPU, the idle task on that CPU calls a few layers of C
> > code before finally leaving the kernel. When KASAN is in use, poisoned
> > shadow is left around for each of the active stack frames, and when
> > shadow call stacks are in use. When shadow call stacks are in use the
> > task's saved SCS SP is left pointing at an arbitrary point within the
> > task's shadow call stack.
> > 
> > When a CPU is offlined than onlined back into the kernel, this stale
> > state can adversely affect execution. Stale KASAN shadow can alias new
> > stackframes and result in bogus KASAN warnings. A stale SCS SP is
> > effectively a memory leak, and prevents a portion of the shadow call
> > stack being used. Across a number of hotplug cycles the idle task's
> > entire shadow call stack can become unusable.
> > 
> > We previously fixed the KASAN issue in commit:
> > 
> >   e1b77c92981a5222 ("sched/kasan: remove stale KASAN poison after hotplug")
> > 
> > ... by removing any stale KASAN stack poison immediately prior to
> > onlining a CPU.
> > 
> > Subsequently in commit:
> > 
> >   f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled")
> > 
> > ... the refactoring left the KASAN and SCS cleanup in one-time idle
> > thread initialization code rather than something invoked prior to each
> > CPU being onlined, breaking both as above.
> > 
> > We fixed SCS (but not KASAN) in commit:
> > 
> >   63acd42c0d4942f7 ("sched/scs: Reset the shadow stack when idle_task_exit")
> > 
> > ... but as this runs in the context of the idle task being offlined it's
> > potentially fragile.
> > 
> > To fix these consistently and more robustly, reset the SCS SP and KASAN
> > shadow of a CPU's idle task immediately before we online that CPU in
> > bringup_cpu(). This ensures the idle task always has a consistent state
> > when it is running, and removes the need to so so when exiting an idle
> > task.
> > 
> > Whenever any thread is created, dup_task_struct() will give the task a
> > stack which is free of KASAN shadow, and initialize the task's SCS SP,
> > so there's no need to specially initialize either for idle thread within
> > init_idle(), as this was only necessary to handle hotplug cycles.
> > 
> > I've tested this with both GCC and clang, with relevant options enabled,
> > offlining and onlining CPUs with:
> > 
> > | while true; do
> > |   for C in /sys/devices/system/cpu/cpu*/online; do
> > |     echo 0 > $C;
> > |     echo 1 > $C;
> > |   done
> > | done
> > 
> > Link: https://lore.kernel.org/lkml/20211012083521.973587-1-woodylin@google.com/
> > Link: https://lore.kernel.org/linux-arm-kernel/YY9ECKyPtDbD9q8q@qian-HP-Z2-SFF-G5-Workstation/
> > Link: https://lore.kernel.org/lkml/20211115113310.35693-1-mark.rutland@arm.com/
> > Fixes: 1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled")
> 
> Ugh, that should be f1a0a376ca0c4ef1 (with a leading 'f'), as in the
> body of the commit message.
> 
> I can fix that up for v3, unless someone's happy to fix that up when
> picking this up.

I just realised I forgot to Cc Ingo, so I'll spin that as v3 and fix the
Cc list.

Mark.

> > Reported-by: Qian Cai <quic_qiancai@...cinc.com>
> > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> > Tested-by: Qian Cai <quic_qiancai@...cinc.com>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Dmitry Vyukov <dvyukov@...gle.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Woody Lin <woodylin@...gle.com>
> > ---
> >  kernel/cpu.c        | 7 +++++++
> >  kernel/sched/core.c | 4 ----
> >  2 files changed, 7 insertions(+), 4 deletions(-)
> > 
> > Since v1 [1]:
> > * Clarify commit message
> > * Fix typos
> > * Accumulate tags
> > 
> > [1] https://lore.kernel.org/lkml/20211115113310.35693-1-mark.rutland@arm.com/
> > 
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index 192e43a87407..407a2568f35e 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -31,6 +31,7 @@
> >  #include <linux/smpboot.h>
> >  #include <linux/relay.h>
> >  #include <linux/slab.h>
> > +#include <linux/scs.h>
> >  #include <linux/percpu-rwsem.h>
> >  #include <linux/cpuset.h>
> >  
> > @@ -588,6 +589,12 @@ static int bringup_cpu(unsigned int cpu)
> >  	int ret;
> >  
> >  	/*
> > +	 * Reset stale stack state from the last time this CPU was online.
> > +	 */
> > +	scs_task_reset(idle);
> > +	kasan_unpoison_task_stack(idle);
> > +
> > +	/*
> >  	 * Some architectures have to walk the irq descriptors to
> >  	 * setup the vector space for the cpu which comes online.
> >  	 * Prevent irq alloc/free across the bringup.
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 3c9b0fda64ac..76f9deeaa942 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -8619,9 +8619,6 @@ void __init init_idle(struct task_struct *idle, int cpu)
> >  	idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
> >  	kthread_set_per_cpu(idle, cpu);
> >  
> > -	scs_task_reset(idle);
> > -	kasan_unpoison_task_stack(idle);
> > -
> >  #ifdef CONFIG_SMP
> >  	/*
> >  	 * It's possible that init_idle() gets called multiple times on a task,
> > @@ -8777,7 +8774,6 @@ void idle_task_exit(void)
> >  		finish_arch_post_lock_switch();
> >  	}
> >  
> > -	scs_task_reset(current);
> >  	/* finish_cpu(), as ran on the BP, will clean up the active_mm state */
> >  }
> >  
> > -- 
> > 2.11.0
> > 

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