Re: [PATCH v2] Reset task stack state in bringup_cpu()

[Mark Rutland <mark.rutland@arm.com>]

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.

Thanks,
Mark.

> Reported-by: Qian Cai <quic_qiancai@quicinc.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> Tested-by: Qian Cai <quic_qiancai@quicinc.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Woody Lin <woodylin@google.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
>