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 16:47:41 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>,
        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>,
        "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 15:45, David Hildenbrand wrote:
> On 25.04.22 03:08, 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?
> 
> I'm not sure if this is worth -stable, but no strong opinion.

I have no strong opinion too. I'm just afraid someone might run into it. But swapoff is
expected to be a rare operation anyway...

> 
> The do_swap_page() part was added in 2005:
> 
> commit b81074800b98ac50b64d4c8d34e8abf0fda5e3d1
> Author: Kirill Korotaev <dev@...ru>
> Date:   Mon May 16 21:53:50 2005 -0700
> 
>     [PATCH] do_swap_page() can map random data if swap read fails
>     
>     There is a bug in do_swap_page(): 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 send SIGBUS in case of error.
>     
>     Signed-Off-By: Kirill Korotaev <dev@...ru>
>     Signed-Off-By: Alexey Kuznetsov <kuznet@....inr.ac.ru>
>     Acked-by: Hugh Dickins <hugh@...itas.com>
>     Signed-off-by: Andrew Morton <akpm@...l.org>
>     Signed-off-by: Linus Torvalds <torvalds@...l.org>
> 
> So the do_swap_page() part has been fixed for quite a while already.

Does this mean only do_swap_page maps random data if swap read fails is observed from that time on?
So this might not be worth -stable as it's never seen more than a decade?

Thanks!

>