linux-kernel - [PATCH urgent] Revert "x86/mm/32: Set NX in __supported_pte

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Message-Id: <59bd15f7f4b56b633a611b7f70876c6d2ad01a98.1461685884.git.luto@kernel.org>
Date:	Tue, 26 Apr 2016 08:52:44 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>, Meelis Roos <mroos@...ux.ee>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH urgent] Revert "x86/mm/32: Set NX in __supported_pte_mask before enabling paging"

This reverts commit 320d25b6a05f8b73c23fc21025d2906ecdd2d4fc.

This change was problematic for a couple of reasons:

1. It missed a some entry points (Xen things and 64-bit native).

2. The entry it changed can be executed more than once.  This isn't
   really a problem, but it conflated per-cpu state setup and global
   state setup.

3. It broke 64-bit non-NX.  64-bit non-NX worked the other way around from
   32-bit -- __supported_pte_mask had NX set initially and was *cleared*
   in x86_configure_nx.  With the patch applied, it never got cleared.

Reported-by: Meelis Roos <mroos@...ux.ee>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/head_32.S | 6 ------
 arch/x86/mm/setup_nx.c    | 5 +++--
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 54cdbd2003fe..af1112980dd4 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -389,12 +389,6 @@ default_entry:
 	/* Make changes effective */
 	wrmsr
 
-	/*
-	 * And make sure that all the mappings we set up have NX set from
-	 * the beginning.
-	 */
-	orl $(1 << (_PAGE_BIT_NX - 32)), pa(__supported_pte_mask + 4)
-
 enable_paging:
 
 /*
diff --git a/arch/x86/mm/setup_nx.c b/arch/x86/mm/setup_nx.c
index 8bea84724a7d..f65a33f505b6 100644
--- a/arch/x86/mm/setup_nx.c
+++ b/arch/x86/mm/setup_nx.c
@@ -32,8 +32,9 @@ early_param("noexec", noexec_setup);
 
 void x86_configure_nx(void)
 {
-	/* If disable_nx is set, clear NX on all new mappings going forward. */
-	if (disable_nx)
+	if (boot_cpu_has(X86_FEATURE_NX) && !disable_nx)
+		__supported_pte_mask |= _PAGE_NX;
+	else
 		__supported_pte_mask &= ~_PAGE_NX;
 }
 
-- 
2.5.5