linux-kernel - Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap read fails

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Date:   Mon, 25 Apr 2022 10:20:23 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "neilb@...e.de" <neilb@...e.de>,
        "david@...hat.com" <david@...hat.com>,
        "apopple@...dia.com" <apopple@...dia.com>,
        "surenb@...gle.com" <surenb@...gle.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap
 read fails

On 2022/4/25 9:08, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Sun, Apr 24, 2022 at 05:11:03PM +0800, Miaohe Lin 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.  In case
>> of error, a special swap entry indicating swap read fails is set to the
>> page table.  So the swapcache page can be freed and the user won't end up
>> with a permanently mounted swap because a sector is bad.  And if the page
>> is accessed later, the user process will be killed so that corrupted data
>> is never consumed.  On the other hand, if the page is never accessed, the
>> user won't even notice it.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> Acked-by: David Hildenbrand <david@...hat.com>
> 
> Hi Miaohe,
> 
> This bug sounds relatively serious to me, and it seems old, so is it worth
> sending to -stable?

This bug is really old but it's never seen yet because swapoff is supposed only to
be done before rebooting the system. But swapoff can happen anytime. Poor guys might
come across it and get wrong data. So I think it's worth sending to -stable.

BTW: This patch should be revised in order to go to the stable version.

Thanks!

> 
> Thanks,
> Naoya Horiguchi
>