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

[Alistair Popple <apopple@nvidia.com>]

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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));