Patch "x86/mm: Do not forbid _PAGE_RW before init for __ro_after_init" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

x86/mm: Do not forbid _PAGE_RW before init for __ro_after_init

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     x86-mm-do-not-forbid-_page_rw-before-init-for-__ro_after_init.patch

and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dave Hansen <[email protected]>
Date: Fri, 6 Apr 2018 13:55:14 -0700
Subject: x86/mm: Do not forbid _PAGE_RW before init for __ro_after_init

From: Dave Hansen <[email protected]>

[ 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.

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

@@ -279,9 +279,11 @@ 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;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/x86-mm-do-not-forbid-_page_rw-before-init-for-__ro_after_init.patch