On  Mon, 15 Nov 2021 at 11:33:10 +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 SCS SP is left pointing at an arbitrary point within the task's
> shadow call stack.
> 
> When an offlines CPU is hotlpugged back into the kernel, this stale
> state can adversely affect the newly onlined CPU. 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 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")
> 
> In commit:
> 
>   f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled")
> 
> ... we broke both KASAN and SCS, with SCS being fixed up 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 offlines it's
> potentially fragile.
> 
> Fix both of these consistently and more robustly by resetting the SCS SP
> and KASAN shadow immediately before we online a CPU. This ensures the
> idle task always has a consistent state, and removes the need to do so
> when initializing an idle task or when unplugging an idle task.
> 
> I've tested this with both GCC and clang, with reelvant options enabled,
> offlining and online 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/
> Fixes: 1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled")
> Reported-by: Qian Cai <quic_qiancai@quicinc.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> 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(-)
> 
> 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);

Hm, in the !CONFIG_GENERIC_SMP_IDLE_THREAD case, idle_thread_get() will
have returned NULL. Won't these then crash?

Admittedly that seems to be *only* for UM, as all other architectures
with SMP seem to set CONFIG_GENERIC_SMP_IDLE_THREAD.

cf.
https://lore.kernel.org/all/5a9d6d4aef78de6fc8a2cfc62922d06617cbe109.camel@infradead.org/
https://lore.kernel.org/all/f59ac9c08122b338bbda137d29013add0e194933.camel@infradead.org/