[PATCH v2 1/2] x86_32/mm: Set NX in __supported_pte_mask before enabling paging

[Andy Lutomirski <luto@kernel.org>]

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@kernel.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 92e2eacb3321..78f5d5907f98 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 (boot_cpu_has(X86_FEATURE_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