[PATCH v4 1/5] mm/swapfile: unuse_pte can map random data if swap read fails

[Miaohe Lin <linmiaohe@huawei.com>]

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>
Acked-by: David Hildenbrand <david@redhat.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 f3ae17b43f20..0c0fed1b348f 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)
 /*
  * 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 - SWP_SWAPIN_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 fe220df499f1..f24775b41880 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(SWP_SWAPIN_ERROR, page_to_pfn(page));
+}
+
+static inline int is_swapin_error_entry(swp_entry_t entry)
+{
+	return swp_type(entry) == SWP_SWAPIN_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 8fc2bc4c9d21..76f371702e99 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1496,7 +1496,8 @@ 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 {
@@ -3742,6 +3743,8 @@ 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 1c3d5b970ddb..226c7d3ba896 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1788,6 +1788,17 @@ 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));
-- 
2.23.0