Re: Re: [PATCH] sched: fix migration to invalid cpu in __set_cpus_allowed_ptr

[shikemeng]

On 13/09/2019 6:09,Valentin Schneider wrote:
>> From 089dbf0216628ac6ae98742ab90725ca9c2bf201 Mon Sep 17 00:00:00 2001
>> From:  <shikemeng@xxxxxxxxxx>
>> Date: Tue, 10 Sep 2019 09:44:58 -0400
>> Subject: [PATCH] sched: fix migration to invalid cpu in 
>> __set_cpus_allowed_ptr
>> 
>> reason: migration to invalid cpu in __set_cpus_allowed_ptr archive 
>> path: patches/euleros/sched
>> 
>
>The above should probably be trimmed from the log.
>

Thanks for reminding me. I will remove this part in a new patch.

>> Oops occur when running qemu on arm64:
>>  Unable to handle kernel paging request at virtual address 
>> ffff000008effe40  Internal error: Oops: 96000007 [#1] SMP  Process 
>> migration/0 (pid: 12, stack limit = 0x00000000084e3736)
>>  pstate: 20000085 (nzCv daIf -PAN -UAO)  pc : 
>> __ll_sc___cmpxchg_case_acq_4+0x4/0x20
>>  lr : move_queued_task.isra.21+0x124/0x298
>>  ...
>>  Call trace:
>>   __ll_sc___cmpxchg_case_acq_4+0x4/0x20
>>   __migrate_task+0xc8/0xe0
>>   migration_cpu_stop+0x170/0x180
>>   cpu_stopper_thread+0xec/0x178
>>   smpboot_thread_fn+0x1ac/0x1e8
>>   kthread+0x134/0x138
>>   ret_from_fork+0x10/0x18
>> 
>> __set_cpus_allowed_ptr will choose an active dest_cpu in affinity mask 
>> to migrage the process if process is not currently running on any one 
>> of the CPUs specified in affinity mask.__set_cpus_allowed_ptr will 
>> choose an invalid dest_cpu(>= nr_cpu_ids, 1024 in my virtual machine) if CPUS in affinity mask are deactived by cpu_down after cpumask_intersects check.
>
>Right, cpumask_any_and() returns >= nr_cpu_ids when there isn't any valid CPU bit set.
>
>> Cpumask_test_cpu of dest_cpu afterwards is overflow and may passes if 
>> corresponding bit is coincidentally set.
>
>Ouch. I was going to say the cpu_active_mask check from is_cpu_allowed() should've stopped the whole thing there, but AFAICT there's no safeguard against > nr_cpu_ids bit accesses. I see CONFIG_DEBUG_PER_CPU_MAPS should fire a warning for such accesses, but we don't enable it by default.
>
>Would it make sense to do something like
>
>	return test_bit(...) && cpu < nr_cpu_ids;
>
>for cpumask_test_cpu()? We still get the warn with the right config, but we prevent sneaky mistakes like this one. And it seems it's not the only one according to:
>
>--
>virtual patch
>virtual report
>
>@nocheck@
>expression E;
>identifier any_func =~ "^cpumask_any";
>position pos;
>@@
>
>E@pos = any_func(...);
>... when != E >= nr_cpu_ids
>    when != E < nr_cpu_ids
>
>@script:python depends on nocheck && report@ p << nocheck.pos; @@ coccilib.report.print_report(p[0], "Missing cpumask_any_*() return value check!")
>---
>
>Some of those seem benign since they are on e.g. cpu_online_mask, some other are somewhat dubious (e.g. deadline.c::dl_task_can_attach()).
>
>As a consequence, kernel will access a invalid rq address associate with the invalid cpu in

It's more thoughtful to add check in cpumask_test_cpu.It can solve this problem and can prevent other potential bugs.I will test it and resend
a new patch.

>> migration_cpu_stop->__migrate_task->move_queued_task and the Oops occurs. Process as follows may trigger the Oops:
>> 1) A process repeatedly bind itself to cpu0 and cpu1 in turn by 
>> calling sched_setaffinity
>> 2) A shell script repeatedly "echo 0 > 
>> /sys/devices/system/cpu/cpu1/online" and "echo 1 > 
>> /sys/devices/system/cpu/cpu1/online" in turn
>> 3) Oops appears if the invalid cpu is set in memory after tested cpumask.
>> 
>> 
>> Change-Id: I9c2f95aecd3da568991b7408397215f26c990e40
>
>- This doesn't respect the 75 char per line limit
>- Change IDs don't belong here (we're not using Gerrit)
>- You're missing a Signed-off-by.
>
>You'll find all the guidelines you need for formatting patches in Documentation/process/submitting-patches.rst.
>

Thanks for the guide.I will read it carefully before send next patch.

>> ---
>>  kernel/sched/core.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 
>> 4b63fef..5181ea9 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1112,7 +1112,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>>     if (cpumask_equal(&p->cpus_allowed, new_mask))
>>         goto out;
>> 
>> -   if (!cpumask_intersects(new_mask, cpu_valid_mask)) {
>> +   dest_cpu = cpumask_any_and(cpu_valid_mask, new_mask);
>> +   if (dest_cpu >= nr_cpu_ids) {
>>         ret = -EINVAL;
>>         goto out;
>>     }
>> @@ -1133,7 +1134,6 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>>     if (cpumask_test_cpu(task_cpu(p), new_mask))
>>         goto out;
>> 
>> -   dest_cpu = cpumask_any_and(cpu_valid_mask, new_mask);>     if (task_running(rq, p) || p->state == TASK_WAKING) {
>>         struct migration_arg arg = { p, dest_cpu };
>>         /* Need help from migration thread: drop lock and wait. */
>> --
>> 1.8.5.6
>> 

Thansks for your review and advises.