linux-kernel - [PATCH 1/2] x86_32/mm: Set NX in __supported_pte

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Date:	Tue, 15 Dec 2015 00:09:29 -0800
From:	Andy Lutomirski <luto@...nel.org>
To:	Pavel Machek <pavel@....cz>, x86@...nel.org
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>, hpa@...or.com,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 1/2] x86_32/mm: Set NX in __supported_pte_mask before enabling paging

There's a short window in which very early mappings can end up with
NX clear because they are created before we've noticed that we have
NX.

It turns out that we detect NX very early, so there's no need to
defer __supported_pte_mask setup.

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, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 6bc9ae24b6d2..57fc3f8c85fd 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -389,6 +389,12 @@ 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 90555bf60aa4..5095ecb0719c 100644
--- a/arch/x86/mm/setup_nx.c
+++ b/arch/x86/mm/setup_nx.c
@@ -31,9 +31,8 @@ early_param("noexec", noexec_setup);
 
 void x86_configure_nx(void)
 {
-	if (cpu_has_nx && !disable_nx)
-		__supported_pte_mask |= _PAGE_NX;
-	else
+	/* If disable_nx is set, clear NX on all new mappings going forward. */
+	if (disable_nx)
 		__supported_pte_mask &= ~_PAGE_NX;
 }
 
-- 
2.5.0

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