linux-kernel - Re: [PATCH] mm/swapfile: unuse_pte can map random data if swap read fails

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Message-Id: <20220404155359.d4867fb8717fe40b5a11647c@linux-foundation.org>
Date:   Mon, 4 Apr 2022 15:53:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/swapfile: unuse_pte can map random data if swap read
 fails

On Fri, 1 Apr 2022 15:29:26 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:

> There is a bug in unuse_pte(): when swap page happens to be unreadable,
> page filled with random data is mapped into user address space. The fix
> is to check for PageUptodate and fail swapoff in case of error.
> 
> ...
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1795,6 +1795,10 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  		ret = 0;
>  		goto out;
>  	}
> +	if (unlikely(!PageUptodate(page))) {
> +		ret = -EIO;
> +		goto out;
> +	}
>  
>  	dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
>  	inc_mm_counter(vma->vm_mm, MM_ANONPAGES);

Failing the swapoff after -EIO seems a bit rude.  The user ends up with
a permanently mounted swap because a sector was bad?

That would be like failing truncate() or close() or umount after -EIO
on a regular file.  Somewhat.

Can we do something better?  Such as shooting down the page anyway and
permitting the swapoff to proceed?  Worst case, just leak the dang page
with an apologetic message.