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

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Date:   Tue, 19 Apr 2022 17:53:03 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, vbabka@...e.cz,
        dhowells@...hat.com, neilb@...e.de, david@...hat.com,
        surenb@...gle.com, minchan@...nel.org, peterx@...hat.com,
        sfr@...b.auug.org.au, rcampbell@...dia.com,
        naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap
 read fails

Also in madvise_free_pte_range() you could just remove the swap entry as it's no
longer needed.

Alistair Popple <apopple@...dia.com> writes:

> Miaohe Lin <linmiaohe@...wei.com> writes:
>
>> 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.
>
> Hi Miaohe,
>
> It seems we're not actually using the pfn that gets stored in the special swap
> entry here. Is my understanding correct? If so I think it would be better to use
> the new PTE markers Peter introduced[1] rather than adding another swap entry
> type.
>
> [1] - <https://lore.kernel.org/linux-mm/20220405014833.14015-1-peterx@redhat.com/>
>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.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)
>>  /*
>>   * PTE markers are used to persist information onto PTEs that are mapped with
>>   * file-backed memories.  As its name "PTE" hints, it should only be applied to
>> @@ -120,7 +124,8 @@ static inline int current_is_kswapd(void)
>>
>>  #define MAX_SWAPFILES \
>>  	((1 << MAX_SWAPFILES_SHIFT) - SWP_DEVICE_NUM - \
>> -	SWP_MIGRATION_NUM - SWP_HWPOISON_NUM - SWP_PTE_MARKER_NUM)
>> +	SWP_MIGRATION_NUM - SWP_HWPOISON_NUM - \
>> +	SWP_PTE_MARKER_NUM - SWAP_READ_ERROR_NUM)
>>
>>  /*
>>   * Magic header for a swap area. The first part of the union is
>> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
>> index fffbba0036f6..d1093384de9f 100644
>> --- a/include/linux/swapops.h
>> +++ b/include/linux/swapops.h
>> @@ -108,6 +108,16 @@ static inline void *swp_to_radix_entry(swp_entry_t entry)
>>  	return xa_mk_value(entry.val);
>>  }
>>
>> +static inline swp_entry_t make_swapin_error_entry(struct page *page)
>> +{
>> +	return swp_entry(SWAP_READ_ERROR, page_to_pfn(page));
>> +}
>> +
>> +static inline int is_swapin_error_entry(swp_entry_t entry)
>> +{
>> +	return swp_type(entry) == SWAP_READ_ERROR;
>> +}
>> +
>>  #if IS_ENABLED(CONFIG_DEVICE_PRIVATE)
>>  static inline swp_entry_t make_readable_device_private_entry(pgoff_t offset)
>>  {
>> diff --git a/mm/memory.c b/mm/memory.c
>> index e6434b824009..34d1d66a05bd 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1476,7 +1476,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>  			/* Only drop the uffd-wp marker if explicitly requested */
>>  			if (!zap_drop_file_uffd_wp(details))
>>  				continue;
>> -		} else if (is_hwpoison_entry(entry)) {
>> +		} else if (is_hwpoison_entry(entry) ||
>> +			   is_swapin_error_entry(entry)) {
>>  			if (!should_zap_cows(details))
>>  				continue;
>>  		} else {
>> @@ -3724,6 +3725,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>  			ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
>>  		} else if (is_hwpoison_entry(entry)) {
>>  			ret = VM_FAULT_HWPOISON;
>> +		} else if (is_swapin_error_entry(entry)) {
>> +			ret = VM_FAULT_SIGBUS;
>>  		} else if (is_pte_marker_entry(entry)) {
>>  			ret = handle_pte_marker(vmf);
>>  		} else {
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 9398e915b36b..95b63f69f388 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1797,6 +1797,17 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>>  		goto out;
>>  	}
>>
>> +	if (unlikely(!PageUptodate(page))) {
>> +		pte_t pteval;
>> +
>> +		dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
>> +		pteval = swp_entry_to_pte(make_swapin_error_entry(page));
>> +		set_pte_at(vma->vm_mm, addr, pte, pteval);
>> +		swap_free(entry);
>> +		ret = 0;
>> +		goto out;
>> +	}
>> +
>>  	/* See do_swap_page() */
>>  	BUG_ON(!PageAnon(page) && PageMappedToDisk(page));
>>  	BUG_ON(PageAnon(page) && PageAnonExclusive(page));