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

Alistair Popple
2022-04-19T08:04:52Z

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

Alistair Popple <apopple@nvidia.com> writes:

> Miaohe Lin <linmiaohe@huawei.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@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)
>> /*
>> * 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));