[PATCH 08/11] x86/mm: do not forbid _PAGE_RW before init for __ro_after_init

From: Dave Hansen
Date: Mon Apr 02 2018 - 13:30:01 EST

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

__ro_after_init data gets stuck in the .rodata section. That's normally
fine because the kernel itself manages the R/W properties.

But, if we run __change_page_attr() on an area which is __ro_after_init,
the .rodata checks will trigger and force the area to be immediately
read-only, even if it is early-ish in boot. This caused problems when
trying to clear the _PAGE_GLOBAL bit for these area in the PTI code:
it cleared _PAGE_GLOBAL like I asked, but also took it up on itself
to clear _PAGE_RW. The kernel then oopses the next time it wrote to
a __ro_after_init data structure.

To fix this, add the kernel_set_to_readonly check, just like we have
for kernel text, just a few lines below in this function.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Nadav Amit <namit@xxxxxxxxxx>

b/arch/x86/mm/pageattr.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff -puN arch/x86/mm/pageattr.c~check-kernel_set_to_readonly arch/x86/mm/pageattr.c
--- a/arch/x86/mm/pageattr.c~check-kernel_set_to_readonly 2018-04-02 10:26:46.648661208 -0700
+++ b/arch/x86/mm/pageattr.c 2018-04-02 10:26:46.652661208 -0700
@@ -298,9 +298,11 @@ static inline pgprot_t static_protection

* The .rodata section needs to be read-only. Using the pfn
- * catches all aliases.
+ * catches all aliases. This also includes __ro_after_init,
+ * so do not enforce until kernel_set_to_readonly is true.
- if (within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT,
+ if (kernel_set_to_readonly &&
+ within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT,
__pa_symbol(__end_rodata) >> PAGE_SHIFT))
pgprot_val(forbidden) |= _PAGE_RW;