Messages in this thread Patch in this message | | | Date | Tue, 23 Nov 2021 11:09:44 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v2] Reset task stack state in bringup_cpu() |
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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@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 > >
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