[PATCH v4 02/15] mm: Make empty_zero_page __ro_after_init

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

The empty zero page is used to back any kernel or user space mapping
that is supposed to remain cleared, and so the page itself is never
supposed to be modified.

So make it __ro_after_init rather than __page_aligned_bss: on most
architectures, this ensures that both the kernel's mapping of it and any
aliases that are accessible via the kernel direct (linear) map are
mapped read-only, and cannot be used (inadvertently or maliciously) to
corrupt the contents of the zero page.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 mm/mm_init.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index f9f8e1af921c..6ca01ed2a5a4 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -57,7 +57,7 @@ unsigned long zero_page_pfn __ro_after_init;
 EXPORT_SYMBOL(zero_page_pfn);
 
 #ifndef __HAVE_COLOR_ZERO_PAGE
-uint8_t empty_zero_page[PAGE_SIZE] __page_aligned_bss;
+uint8_t empty_zero_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE);
 EXPORT_SYMBOL(empty_zero_page);
 
 struct page *__zero_page __ro_after_init;
-- 
2.54.0.rc2.544.gc7ae2d5bb8-goog