linux-kernel - [PATCH 2/3] x86/boot: Fix another __read

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Date:   Wed, 28 Sep 2016 12:34:15 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     david@...giorato.net, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 2/3] x86/boot: Fix another __read_cr4() case on 486

The condition for reading CR4 was wrong: there are some CPUs with
CPUID but not CR4.  Rather than trying to make the condition exact,
using __read_cr4_safe().

Reported-by: david@...giorato.net
Fixes: 18bc7bd523e0 ("x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features directly")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/setup.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 0fa60f5f5a16..5930a4d191b4 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1137,9 +1137,13 @@ void __init setup_arch(char **cmdline_p)
 	 * auditing all the early-boot CR4 manipulation would be needed to
 	 * rule it out.
 	 */
-	if (boot_cpu_data.cpuid_level >= 0)
-		/* A CPU has %cr4 if and only if it has CPUID. */
-		mmu_cr4_features = __read_cr4();
+	if (boot_cpu_data.cpuid_level >= 0) {
+		/*
+		 * CPUs without CPUID don't have CR4.  CPUs with CPUID
+		 * usually have CR4.
+		 */
+		mmu_cr4_features = __read_cr4_safe();
+	}
 
 	memblock_set_current_limit(get_max_mapped());
 
-- 
2.7.4