On Mon, Nov 15, 2021 at 10:44:11AM +0000, Mark Rutland wrote:

Hi,

On Fri, Nov 12, 2021 at 11:50:16PM -0500, Qian Cai wrote:

quoted

FYI, running CPU soft-hotplug with KASAN on arm64 defconfig will
always trigger a stack-out-of-bounds below. I am not right sure where
exactly KASAN pointed at, so I am just doing the brute-force
bisect. The progress so far:

From below it looks like this is on linux-next; I can reproduce this on
v5.16-rc1 using your config, when hotplugging CPU0 back in.

We used to have issues with stale poison being left on the stack across a
hotplug, and we fixed that with commit:

  e1b77c92981a5222 ("sched/kasan: remove stale KASAN poison after hotplug")

... but it looks like we no longer call init_idle() for each hotplug since commit:

  f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled")

... and so don't get the kasan_unpoison_task_stack() call which we want when
bringing up a CPU, which we used to get by way of idle_thread_get() calling init_idle().

Adding a call to kasan_unpoison_task_stack(idle) within bringup_cpu() gets rid
of that, and I reckon we want that explciitly *somewhere* on the CPU bringup
path.

FWIW I sent that out as a patch:

https://lore.kernel.org/linux-arm-kernel/20211115113310.35693-1-mark.rutland@arm.com/ (local)

Thanks,
Mark.

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