From: Mark Rutland mark.rutland@arm.com
[ Upstream commit dce1ca0525bfdc8a69a9343bc714fbc19a2f04b3 ]
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 (SCS) 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 on arm64 with:
* gcc 11.1.0, defconfig +KASAN_INLINE, KASAN_STACK * clang 12.0.0, defconfig +KASAN_INLINE, KASAN_STACK, SHADOW_CALL_STACK
... 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
Fixes: f1a0a376ca0c4ef1 ("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 Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Valentin Schneider valentin.schneider@arm.com Tested-by: Qian Cai quic_qiancai@quicinc.com Link: https://lore.kernel.org/lkml/20211115113310.35693-1-mark.rutland@arm.com/ Signed-off-by: Sasha Levin sashal@kernel.org --- 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 192e43a874076..407a2568f35eb 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>
@@ -587,6 +588,12 @@ static int bringup_cpu(unsigned int cpu) struct task_struct *idle = idle_thread_get(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. diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 779f27a4b46ac..6f4625f8276f1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -8641,9 +8641,6 @@ 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, @@ -8799,7 +8796,6 @@ 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 */ }