linux-kernel - [PATCH 16/16] x86: Restrict MSR access when the kernel is locked down

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Date:   Wed, 16 Nov 2016 21:49:12 +0000
From:   David Howells <dhowells@...hat.com>
To:     keyrings@...r.kernel.org
Cc:     matthew.garrett@...ula.com, linux-efi@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, dhowells@...hat.com,
        linux-security-module@...r.kernel.org
Subject: [PATCH 16/16] x86: Restrict MSR access when the kernel is locked
 down

From: Matthew Garrett <matthew.garrett@...ula.com>

Writing to MSRs should not be allowed if the kernel is locked down, since
it could lead to execution of arbitrary code in kernel mode.  Based on a
patch by Kees Cook.

Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Matthew Garrett <matthew.garrett@...ula.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 arch/x86/kernel/msr.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 7f3550acde1b..b875e54bdbf7 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -39,6 +39,7 @@
 #include <linux/notifier.h>
 #include <linux/uaccess.h>
 #include <linux/gfp.h>
+#include <linux/security.h>
 
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
@@ -83,6 +84,9 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
 	int err = 0;
 	ssize_t bytes = 0;
 
+	if (kernel_is_locked_down())
+		return -EPERM;
+
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
 
@@ -130,6 +134,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 			err = -EBADF;
 			break;
 		}
+		if (kernel_is_locked_down()) {
+			err = -EPERM;
+			break;
+		}
 		if (copy_from_user(&regs, uregs, sizeof regs)) {
 			err = -EFAULT;
 			break;