[4.16,051/272] x86/mm: Do not forbid _PAGE_RW before init for __ro_after_init

Commit Message

Greg KH May 28, 2018, 10:01 a.m. UTC

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dave Hansen <[e-mail]>

[ Upstream commit 639d6aafe437a7464399d2a77d006049053df06f ]

__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 <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Sasha Levin <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
---
 arch/x86/mm/pageattr.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Patch

--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -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;