Re: [PATCH] Reset task stack state in bringup_cpu()
From: Valentin Schneider <hidden>
Date: 2021-11-15 12:18:23
Also in:
lkml
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. 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. When an offlines CPU is hotlpugged back into the kernel, this stale
^^^^^^^^ ^^^^^^^^^^
offlined? hotplugged
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 ^^^^^^^^
offlined
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,
^^^^^^^^
relevant
offlining and online CPUs with:
^^^^^^
onlining
| 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/ (local) Link: https://lore.kernel.org/linux-arm-kernel/YY9ECKyPtDbD9q8q@qian-HP-Z2-SFF-G5-Workstation/ (local) Fixes: 1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") Reported-by: Qian Cai <redacted> 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 <redacted> Cc: Will Deacon <will@kernel.org> Cc: Woody Lin <redacted>
Reviewed-by: Valentin Schneider <redacted>
quoted hunk ↗ jump to hunk
--- 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); + + /* * 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); -
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.
quoted hunk ↗ jump to hunk
#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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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