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> --- Hi, CONFIG_PAGE_TABLE_CHECK is pretty explicit about what it checks (and doesn't mention the zero page), but this condition seems to fit with the general category of "pages mapped wrongly to userspace". I added it locally to help me debug something. Maybe it's more widely useful. 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); base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5 -- 2.17.1
