[PATCH 1/3] mm/page_table_check: Check writable zero page in page table check

[Pasha Tatashin]

From: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

The zero page should remain all zero, so that it can be mapped as
read-only for read faults of memory that should be zeroed. If it is ever
mapped writable to userspace, it could become non-zero and so other apps
would unexpectedly get non-zero data. So the zero page should never be
mapped writable to userspace. Check for this condition in
page_table_check_set().

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 mm/page_table_check.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index e2062748791a..665ece0d55d4 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -102,6 +102,8 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 	if (!pfn_valid(pfn))
 		return;
 
+	BUG_ON(is_zero_pfn(pfn) && rw);
+
 	page = pfn_to_page(pfn);
 	page_ext = lookup_page_ext(page);
 	anon = PageAnon(page);
-- 
2.37.2.789.g6183377224-goog