Re: [PATCH v2] ACPI: APEI: do not add task_work to kernel thread to avoid memory leak

[Rafael J. Wysocki]

On Sat, Sep 24, 2022 at 9:50 AM Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx> wrote:
>
> If an error is detected as a result of user-space process accessing a
> corrupt memory location, the CPU may take an abort. Then the platform
> firmware reports kernel via NMI like notifications, e.g. NOTIFY_SEA,
> NOTIFY_SOFTWARE_DELEGATED, etc.
>
> For NMI like notifications, commit 7f17b4a121d0 ("ACPI: APEI: Kick the
> memory_failure() queue for synchronous errors") keep track of whether
> memory_failure() work was queued, and make task_work pending to flush out
> the queue so that the work is processed before return to user-space.
>
> The code use init_mm to check whether the error occurs in user space:
>
>     if (current->mm != &init_mm)
>
> The condition is always true, becase _nobody_ ever has "init_mm" as a real
> VM any more.
>
> In addition to abort, errors can also be signaled as asynchronous
> exceptions, such as interrupt and SError. In such case, the interrupted
> current process could be any kind of thread. When a kernel thread is
> interrupted, the work ghes_kick_task_work deferred to task_work will never
> be processed because entry_handler returns to call ret_to_kernel() instead
> of ret_to_user(). Consequently, the estatus_node alloced from
> ghes_estatus_pool in ghes_in_nmi_queue_one_entry() will not be freed.
> After around 200 allocations in our platform, the ghes_estatus_pool will
> run of memory and ghes_in_nmi_queue_one_entry() returns ENOMEM. As a
> result, the event failed to be processed.
>
>     sdei: event 805 on CPU 113 failed with error: -2
>
> Finally, a lot of unhandled events may cause platform firmware to exceed
> some threshold and reboot.
>
> The condition should generally just do
>
>     if (current->mm)
>
> as described in active_mm.rst documentation.
>
> Then if an asynchronous error is detected when a kernel thread is running,
> (e.g. when detected by a background scrubber), do not add task_work to it
> as the original patch intends to do.
>
> Fixes: 7f17b4a121d0 ("ACPI: APEI: Kick the memory_failure() queue for synchronous errors")
> Signed-off-by: Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx>

I need the APEI code reviewers to tell me that this is all OK.

> ---
> changes since v1:
> - add description the side effect and give more details
>
>  drivers/acpi/apei/ghes.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index d91ad378c00d..80ad530583c9 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -985,7 +985,7 @@ static void ghes_proc_in_irq(struct irq_work *irq_work)
>                                 ghes_estatus_cache_add(generic, estatus);
>                 }
>
> -               if (task_work_pending && current->mm != &init_mm) {
> +               if (task_work_pending && current->mm) {
>                         estatus_node->task_work.func = ghes_kick_task_work;
>                         estatus_node->task_work_cpu = smp_processor_id();
>                         ret = task_work_add(current, &estatus_node->task_work,
> --
> 2.20.1.12.g72788fdb
>