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Date:   Wed, 14 Sep 2022 15:35:42 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Shuai Xue <xueshuai@linux.alibaba.com>
Cc:     naoya.horiguchi@nec.com, linmiaohe@huawei.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, cuibixuan@linux.alibaba.com,
        baolin.wang@linux.alibaba.com, zhuo.song@linux.alibaba.com,
        Huang Ying <ying.huang@intel.com>
Subject: Re: [PATCH] mm,hwpoison: check mm when killing accessing process
Message-Id: <20220914153542.285f870f728c6129a479a69d@linux-foundation.org>
In-Reply-To: <20220914064935.7851-1-xueshuai@linux.alibaba.com>
References: <20220914064935.7851-1-xueshuai@linux.alibaba.com>
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On Wed, 14 Sep 2022 14:49:35 +0800 Shuai Xue <xueshuai@linux.alibaba.com> wrote:

> The GHES code calls memory_failure_queue() from IRQ context to queue work
> into workqueue and schedule it on the current CPU. Then the work is
> processed in memory_failure_work_func() by kworker and calls
> memory_failure().
> 
> When a page is already poisoned, commit a3f5d80ea401 ("mm,hwpoison: send
> SIGBUS with error virutal address") make memory_failure() call
> kill_accessing_process() that:
> 
>     - holds mmap locking of current->mm
>     - does pagetable walk to find the error virtual address
>     - and sends SIGBUS to the current process with error info.
> 
> However, the mm of kworker is not valid. Therefore, check mm when killing
> accessing process.

Thanks.

When fixing a bug, please always describe the user-visible effects of
tha bug.  I'm thinking "null pointer deref crashes the kernel".

> Fixes: a3f5d80ea401 ("mm,hwpoison: send SIGBUS with error virutal address")
> Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>

I'll add cc:stable.

> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -743,6 +743,9 @@ static int kill_accessing_process(struct task_struct *p, unsigned long pfn,
>  	};
>  	priv.tk.tsk = p;
>  
> +	if (!p->mm)
> +		return -EFAULT;
> +
>  	mmap_read_lock(p->mm);
>  	ret = walk_page_range(p->mm, 0, TASK_SIZE, &hwp_walk_ops,
>  			      (void *)&priv);
> @@ -751,6 +754,7 @@ static int kill_accessing_process(struct task_struct *p, unsigned long pfn,
>  	else
>  		ret = 0;
>  	mmap_read_unlock(p->mm);
> +
>  	return ret > 0 ? -EHWPOISON : -EFAULT;
>  }

This is an unrelated change which doesn't appear to match the style in
memory-failure.c, so I'll drop this hunk.