Messages in this thread Patch in this message | | | Date | Mon, 15 Nov 2021 14:09:27 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] Reset task stack state in bringup_cpu() |
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On Mon, Nov 15, 2021 at 12:16:14PM +0000, Valentin Schneider wrote: > > Hi Mark, > > Thanks for tackling this and glueing the pieces back together. LGTM, though > I couldn't stop myself from playing changelog police - I also have a > question/comment wrt the BP.
I'll go fix the various typos for v2; replies to the more substantial comments below.
> On 15/11/21 11:33, 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.
> > Fix both of these consistently and more robustly by resetting the SCS SP > > and KASAN shadow immediately before we online a CPU.
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Thanks!
> > 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); > > - > > So those are no longer invoked for the BP during bootup (via sched_init()); > that looks OK for KASAN per: > > e1b77c92981a ("sched/kasan: remove stale KASAN poison after hotplug") > > I didn't find any explicit commit for SCS but from the looks of > arm64/include/asm/thread_info.h we seem to be initializing things > correctly, so IIUC the removed hunk wasn't actually necessary for the BP's > first boot.
Correct. For the init task:
* The KASAN shadow starts out empty, and there's no poison to remove.
* The saved SCS SP is initialized statically as part of INIT_THREAD_INFO(), via INIT_SCS().
... so that requires no special care.
For every task created thereafter (including idle threads):
* dup_task_struct() allocates the new task's stack via alloc_thread_stack_node(), which either explicitly removes KASAN poison from the shadow of a cached stack, or acquires a stack via __vmalloc_node_range(), whose shadow starts out empty. * dup_task_struct() calls scs_prepare() which allocates the task's shadow stack and initializes the SCS SP for the task.
The idle threads get created via fork_idle(), which calls copy_process() (and therefore dup_task_struct) to allocate the idle thread, then calls init_idle() on the result.
Thanks Mark.
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