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 09:49:43 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "ying.huang@...el.com" <ying.huang@...el.com>,
        Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, apopple@...dia.com, surenb@...gle.com,
        minchan@...nel.org, peterx@...hat.com, sfr@...b.auug.org.au,
        naoya.horiguchi@....com, linux-mm@...ck.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 25.04.22 09:41, ying.huang@...el.com wrote:
> Hi, Miaohe,
> 
> On Sun, 2022-04-24 at 17:11 +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>
>> ---
>>  include/linux/swap.h    |  7 ++++++-
>>  include/linux/swapops.h | 10 ++++++++++
>>  mm/memory.c             |  5 ++++-
>>  mm/swapfile.c           | 11 +++++++++++
>>  4 files changed, 31 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 5553189d0215..b82c196d8867 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -55,6 +55,10 @@ static inline int current_is_kswapd(void)
>>   * actions on faults.
>>   */
>>
>> +#define SWP_SWAPIN_ERROR_NUM 1
>> +#define SWP_SWAPIN_ERROR     (MAX_SWAPFILES + SWP_HWPOISON_NUM + \
>> +			     SWP_MIGRATION_NUM + SWP_DEVICE_NUM + \
>> +			     SWP_PTE_MARKER_NUM)
>>
>>
> 
> It appears wasteful to use another swap device number. 

Do we really care?

We currently use 5 bits for swap types, so we have a total of 32.

SWP_HWPOISON_NUM -> 1
SWP_MIGRATION_NUM -> 3
SWP_PTE_MARKER_NUM -> 1
SWP_DEVICE_NUM -> 4
SWP_SWAPIN_ERROR_NUM -> 1

Which would leave us with 32 - 10 = 22 swap devices. IMHO that's plenty
for real life scenarios.

I'd prefer reworking this when we really run into trouble (and we could
think about using more bits for applicable architectures then, for
select 64bit architectures it might be fairly easily possible).

-- 
Thanks,

David / dhildenb