linux-kernel - [tip:x86/pti] x86/mm: Do not forbid _PAGE_RW before init for __ro_after

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Date:   Mon, 9 Apr 2018 10:15:09 -0700
From:   tip-bot for Dave Hansen <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     luto@...nel.org, gregkh@...uxfoundation.org, namit@...are.com,
        hpa@...or.com, dan.j.williams@...el.com,
        dave.hansen@...ux.intel.com, tglx@...utronix.de,
        dwmw2@...radead.org, torvalds@...ux-foundation.org,
        hughd@...gle.com, arjan@...ux.intel.com, keescook@...omium.org,
        aarcange@...hat.com, mingo@...nel.org, bp@...en8.de,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com, jgross@...e.com,
        peterz@...radead.org
Subject: [tip:x86/pti] x86/mm: Do not forbid _PAGE_RW before init for
 __ro_after_init

Commit-ID:  efad2b4151521c944e405272035a673c74125c65
Gitweb:     https://git.kernel.org/tip/efad2b4151521c944e405272035a673c74125c65
Author:     Dave Hansen <dave.hansen@...ux.intel.com>
AuthorDate: Fri, 6 Apr 2018 13:55:14 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 9 Apr 2018 18:27:34 +0200

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

__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@...ux.intel.com>
Acked-by: Kees Cook <keescook@...omium.org>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: David Woodhouse <dwmw2@...radead.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Nadav Amit <namit@...are.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-mm@...ck.org
Link: http://lkml.kernel.org/r/20180406205514.8D898241@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/mm/pageattr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 968f51a2e39b..a7324045d87d 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -298,9 +298,11 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
 
 	/*
 	 * 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;