Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap read fails

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/4/19 15:37, David Hildenbrand wrote:
> On 16.04.22 05:05, 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@huawei.com>
>> ---
>> v2:
>>   use special swap entry to avoid permanently mounted swap
>>   free the bad page in swapcache
>> ---
>>  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 d112434f85df..03c576111737 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 SWAP_READ_ERROR_NUM 1
>> +#define SWAP_READ_ERROR     (MAX_SWAPFILES + SWP_HWPOISON_NUM + \
>> +			     SWP_MIGRATION_NUM + SWP_DEVICE_NUM + \
>> +			     SWP_PTE_MARKER_NUM)
> 
> Does anything speak against reusing the hwpoison marker? At least from a
> program POV it's similar "the previously well defined content at this
> user space address is no longer readable/writable".

Looks like a good idea. :)

> 
> I recall that we can just set the pfn to 0 for the hwpoison marker.
> 
> There is e.g., check_hwpoisoned_entry() and it just stops if it finds
> "pfn=0".

Do you mean that we should set the pfn to 0 for the hwpoison marker so that we can
distinguish swapin error case from real hwpoison case?

> 
> 

Will try to do this in next version. Thanks a lot!