Message-ID: <20241017105621.00000c1e@Huawei.com>
Date: Thu, 17 Oct 2024 10:56:21 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Shuai Xue <xueshuai@...ux.alibaba.com>
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Subject: Re: [PATCH v14 3/3] ACPI: APEI: handle synchronous exceptions in
 task work

On Mon, 14 Oct 2024 16:42:40 +0800
Shuai Xue <xueshuai@...ux.alibaba.com> wrote:

> The memory uncorrected error could be signaled by asynchronous interrupt
> (specifically, SPI in arm64 platform), e.g. when an error is detected by
> a background scrubber, or signaled by synchronous exception
> (specifically, data abort excepction in arm64 platform), e.g. when a CPU

exception

> tries to access a poisoned cache line. Currently, both synchronous and
> asynchronous error use memory_failure_queue() to schedule
> memory_failure() exectute in kworker context.
memory_failure() to execute in kworker context.

(spell check in general)
> 
> As a result, when a user-space process is accessing a poisoned data, a
> data abort is taken and the memory_failure() is executed in the kworker
> context:
context, memory_failure():
//No subject of the following two bullets otherwise.
> 
>   - will send wrong si_code by SIGBUS signal in early_kill mode, and
>   - can not kill the user-space in some cases resulting a synchronous
>     error infinite loop
> 
> Issue 1: send wrong si_code in early_kill mode
> 
> Since commit a70297d22132 ("ACPI: APEI: set memory failure flags as
> MF_ACTION_REQUIRED on synchronous events")', the flag MF_ACTION_REQUIRED
> could be used to determine whether a synchronous exception occurs on
> ARM64 platform.  When a synchronous exception is detected, the kernel is
> expected to terminate the current process which has accessed poisoned
> page. This is done by sending a SIGBUS signal with an error code
> BUS_MCEERR_AR, indicating an action-required machine check error on
> read.
> 
> However, when kill_proc() is called to terminate the processes who have
> the poisoned page mapped, it sends the incorrect SIGBUS error code
> BUS_MCEERR_AO because the context in which it operates is not the one
> where the error was triggered.
> 
> To reproduce this problem:
> 
>   #sysctl -w vm.memory_failure_early_kill=1
>   vm.memory_failure_early_kill = 1
> 
>   # STEP2: inject an UCE error and consume it to trigger a synchronous error
>   #einj_mem_uc single
>   0: single   vaddr = 0xffffb0d75400 paddr = 4092d55b400
>   injecting ...
>   triggering ...
>   signal 7 code 5 addr 0xffffb0d75000
>   page not present
>   Test passed
> 
> The si_code (code 5) from einj_mem_uc indicates that it is BUS_MCEERR_AO
> error and it is not fact.
> 
> After this patch:
> 
>   # STEP1: enable early kill mode
>   #sysctl -w vm.memory_failure_early_kill=1
>   vm.memory_failure_early_kill = 1
>   # STEP2: inject an UCE error and consume it to trigger a synchronous error
>   #einj_mem_uc single
>   0: single   vaddr = 0xffffb0d75400 paddr = 4092d55b400
>   injecting ...
>   triggering ...
>   signal 7 code 4 addr 0xffffb0d75000
>   page not present
>   Test passed
> 
> The si_code (code 4) from einj_mem_uc indicates that it is BUS_MCEERR_AR
> error as we expected.
> 
> Issue 2: a synchronous error infinite loop
> 
> If a user-space process, e.g. devmem, a poisoned page which has been set

devmem accesses a poisoned page for which the HWPoison flag is set

> HWPosion flag, kill_accessing_process() is called to send SIGBUS to the
> current processs with error info. Because the memory_failure() is
> executed in the kworker contex, it will just do nothing but return

context

> EFAULT. So, devmem will access the posioned page and trigger an
> excepction again, resulting in a synchronous error infinite loop. Such

exception

> loop may cause platform firmware to exceed some threshold and reboot
> when Linux could have recovered from this error.
> 
> To reproduce this problem:
> 
>   # STEP 1: inject an UCE error, and kernel will set HWPosion flag for related page
>   #einj_mem_uc single
>   0: single   vaddr = 0xffffb0d75400 paddr = 4092d55b400
>   injecting ...
>   triggering ...
>   signal 7 code 4 addr 0xffffb0d75000
>   page not present
>   Test passed
> 
>   # STEP 2: access the same page and it will trigger a synchronous error infinite loop
>   devmem 0x4092d55b400
> 
> To fix above two issues, queue memory_failure() as a task_work so that it runs in
> the context of the process that is actually consuming the poisoned data.
> 
> Signed-off-by: Shuai Xue <xueshuai@...ux.alibaba.com>
> Tested-by: Ma Wupeng <mawupeng1@...wei.com>
> Reviewed-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> Reviewed-by: Xiaofei Tan <tanxiaofei@...wei.com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
One other trivial comment inline.

Whilst this also looks fine to me, there are others who (hopefully)
understand these paths better than me.  With that said.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

> ---
>  drivers/acpi/apei/ghes.c | 78 +++++++++++++++++++++++-----------------
>  include/acpi/ghes.h      |  3 --
>  include/linux/mm.h       |  1 -
>  mm/memory-failure.c      | 13 -------
>  4 files changed, 45 insertions(+), 50 deletions(-)
> 
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index f2ee28c44d7a..95e9520eb803 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -467,28 +467,42 @@ static void ghes_clear_estatus(struct ghes *ghes,
>  }
>  
>  /*
> - * Called as task_work before returning to user-space.
> - * Ensure any queued work has been done before we return to the context that
> - * triggered the notification.
> + * struct ghes_task_work - for synchronous RAS event
> + *
> + * @twork:                callback_head for task work
> + * @pfn:                  page frame number of corrupted page
> + * @flags:                work control flags
> + *
> + * Structure to pass task work to be handled before
> + * returning to user-space via task_work_add().
>   */
> -static void ghes_kick_task_work(struct callback_head *head)
> +struct ghes_task_work {
> +	struct callback_head twork;
> +	u64 pfn;
> +	int flags;
> +};
> +
> +static void memory_failure_cb(struct callback_head *twork)
>  {
> -	struct acpi_hest_generic_status *estatus;
> -	struct ghes_estatus_node *estatus_node;
> -	u32 node_len;
> +	struct ghes_task_work *twcb = container_of(twork, struct ghes_task_work, twork);
> +	unsigned long pfn = twcb->pfn;

This local variable is not used consistently.  I'd just
drop it in favor of always accessing via twcb->pfn

> +	int ret;
>  
> -	estatus_node = container_of(head, struct ghes_estatus_node, task_work);
> -	if (IS_ENABLED(CONFIG_ACPI_APEI_MEMORY_FAILURE))
> -		memory_failure_queue_kick(estatus_node->task_work_cpu);
> +	ret = memory_failure(twcb->pfn, twcb->flags);

> +	gen_pool_free(ghes_estatus_pool, (unsigned long)twcb, sizeof(*twcb));
>  
> -	estatus = GHES_ESTATUS_FROM_NODE(estatus_node);
> -	node_len = GHES_ESTATUS_NODE_LEN(cper_estatus_len(estatus));
> -	gen_pool_free(ghes_estatus_pool, (unsigned long)estatus_node, node_len);
> +	if (!ret || ret == -EHWPOISON || ret == -EOPNOTSUPP)
> +		return;
> +
> +	pr_err("%#lx: Sending SIGBUS to %s:%d due to hardware memory corruption\n",
> +			pfn, current->comm, task_pid_nr(current));
> +	force_sig(SIGBUS);
>  }

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