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Mark Rutland dce1ca0525 sched/scs: Reset task stack state in bringup_cpu() 
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:

  e1b77c9298 ("sched/kasan: remove stale KASAN poison after hotplug")

... by removing any stale KASAN stack poison immediately prior to
onlining a CPU.

Subsequently in commit:

  f1a0a376ca ("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:

  63acd42c0d ("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: f1a0a376ca ("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/
2021-11-24 12:20:27 +01:00
..
autogroup.c sched/fair: Prevent dead task groups from regaining cfs_rq's 2021-11-11 13:09:33 +01:00
autogroup.h
clock.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
completion.c
core.c sched/scs: Reset task stack state in bringup_cpu() 2021-11-24 12:20:27 +01:00
core_sched.c kernel.sys.v5.16 2021-11-10 16:10:47 -08:00
cpuacct.c sched: Wrap rq::lock access 2021-05-12 11:43:26 +02:00
cpudeadline.c sched,rt: Use the full cpumask for balancing 2020-11-10 18:39:00 +01:00
cpudeadline.h
cpufreq.c
cpufreq_schedutil.c cpufreq: schedutil: Use kobject release() method to free sugov_tunables 2021-08-06 15:34:55 +02:00
cpupri.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
cpupri.h sched/cpupri: Add CPUPRI_HIGHER 2020-10-29 11:00:30 +01:00
cputime.c Scheduler updates for this cycle are: 2021-04-28 13:33:57 -07:00
deadline.c sched/dl: Support schedstats for deadline sched class 2021-10-05 15:51:53 +02:00
debug.c sched: Fix DEBUG && !SCHEDSTATS warn 2021-10-06 10:30:57 +02:00
fair.c sched/fair: Prevent dead task groups from regaining cfs_rq's 2021-11-11 13:09:33 +01:00
features.h sched: Disable TTWU_QUEUE on RT 2021-10-05 15:52:12 +02:00
idle.c sched/idle: Make the idle timer expire in hard interrupt context 2021-09-09 10:36:16 +02:00
isolation.c sched/isolation: Reconcile rcu_nocbs= and nohz_full= 2021-05-13 14:12:47 +02:00
loadavg.c sched: Make multiple runqueue task counters 32-bit 2021-05-12 21:34:17 +02:00
Makefile sched: Disable -Wunused-but-set-variable 2021-10-15 11:25:15 +02:00
membarrier.c sched/membarrier: fix missing local execution of ipi_sync_rq_state() 2021-03-06 12:40:21 +01:00
pelt.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
pelt.h Merge branch 'sched/urgent' into sched/core, to resolve conflicts 2021-06-18 11:31:25 +02:00
psi.c Merge branch 'for-5.14' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup 2021-07-01 17:22:14 -07:00
rt.c sched/fair: Prevent dead task groups from regaining cfs_rq's 2021-11-11 13:09:33 +01:00
sched-pelt.h
sched.h sched/fair: Prevent dead task groups from regaining cfs_rq's 2021-11-11 13:09:33 +01:00
smp.h
stats.c sched: Introduce task block time in schedstats 2021-10-05 15:51:48 +02:00
stats.h sched: Make schedstats helpers independent of fair sched class 2021-10-05 15:51:47 +02:00
stop_task.c sched: Make struct sched_statistics independent of fair sched class 2021-10-05 15:51:45 +02:00
swait.c
topology.c Merge branch 'akpm' (patches from Andrew) 2021-11-06 14:08:17 -07:00
wait.c rq-qos: fix missed wake-ups in rq_qos_throttle try two 2021-06-08 15:12:57 -06:00
wait_bit.c