linux-kernel - [PATCH] x86/mm: keep __default_kernel_pte_mask in sync with __supported_pte

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Date:   Wed, 27 May 2020 17:53:39 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: [PATCH] x86/mm: keep __default_kernel_pte_mask in sync with
 __supported_pte_mask

Both masks get applied in the process of e.g. set_fixmap() - the former
through use of PAGE_KERNEL, the latter by use of massage_pgprot(). Hence
forever since the introduction of the former there was a time window
(between x86_configure_nx() and the syncing of the two in
probe_page_size_mask(), as called from init_mem_mapping()) where fixmap
mappings would get established without NX set. For a 32-bit kernel
running in PV mode under Xen this meant a W+X mapping (and associated
warning) for its shared info page mapping established in
xen_pv_init_platform().

Signed-off-by: Jan Beulich <jbeulich@...e.com>

--- a/arch/x86/mm/setup_nx.c
+++ b/arch/x86/mm/setup_nx.c
@@ -33,10 +33,13 @@ early_param("noexec", noexec_setup);

 void x86_configure_nx(void)
 {
-	if (boot_cpu_has(X86_FEATURE_NX) && !disable_nx)
+	if (boot_cpu_has(X86_FEATURE_NX) && !disable_nx) {
 		__supported_pte_mask |= _PAGE_NX;
-	else
+		__default_kernel_pte_mask |= _PAGE_NX;
+	} else {
 		__supported_pte_mask &= ~_PAGE_NX;
+		__default_kernel_pte_mask &= ~_PAGE_NX;
+	}
 }

 void __init x86_report_nx(void)