[PATCH v2 4/4] mm: page_table_check: Ensure user pages are not slab pages

[Ruihan Li <lrh2000@pku.edu.cn>]

The current uses of PageAnon in page table check functions can lead to
type confusion bugs between struct page and slab [1], if slab pages are
accidentally mapped into the user space. This is because slab reuses the
bits in struct page to store its internal states, which renders PageAnon
ineffective on slab pages.

Since slab pages are not expected to be mapped into the user space, this
patch adds BUG_ON(PageSlab(page)) checks to make sure that slab pages
are not inadvertently mapped. Otherwise, there must be some bugs in the
kernel.

Reported-by: syzbot+fcf1a817ceb50935ce99@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/000000000000258e5e05fae79fc1@google.com/ [1]
Fixes: df4e817b7108 ("mm: page table check")
Cc: <stable@vger.kernel.org> # 5.17
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
---
 include/linux/page-flags.h | 6 ++++++
 mm/page_table_check.c      | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 1c68d67b8..92a2063a0 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -617,6 +617,12 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
  * Please note that, confusingly, "page_mapping" refers to the inode
  * address_space which maps the page from disk; whereas "page_mapped"
  * refers to user virtual address space into which the page is mapped.
+ *
+ * For slab pages, since slab reuses the bits in struct page to store its
+ * internal states, the page->mapping does not exist as such, nor do these
+ * flags below.  So in order to avoid testing non-existent bits, please
+ * make sure that PageSlab(page) actually evaluates to false before calling
+ * the following functions (e.g., PageAnon).  See mm/slab.h.
  */
 #define PAGE_MAPPING_ANON	0x1
 #define PAGE_MAPPING_MOVABLE	0x2
diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 25d8610c0..f2baf97d5 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -71,6 +71,8 @@ static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
 
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
+
+	BUG_ON(PageSlab(page));
 	anon = PageAnon(page);
 
 	for (i = 0; i < pgcnt; i++) {
@@ -107,6 +109,8 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
+
+	BUG_ON(PageSlab(page));
 	anon = PageAnon(page);
 
 	for (i = 0; i < pgcnt; i++) {
@@ -133,6 +137,8 @@ void __page_table_check_zero(struct page *page, unsigned int order)
 	struct page_ext *page_ext;
 	unsigned long i;
 
+	BUG_ON(PageSlab(page));
+
 	page_ext = page_ext_get(page);
 	BUG_ON(!page_ext);
 	for (i = 0; i < (1ul << order); i++) {
-- 
2.40.1