linux-kernel - [PATCH 10/12] x86: Restrict MSR access when trusted

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Date:	Fri, 13 Mar 2015 11:38:26 -1000
From:	Matthew Garrett <matthew.garrett@...ula.com>
To:	linux-security-module@...r.kernel.org
Cc:	james.l.morris@...cle.com, serge@...lyn.com,
	linux-kernel@...r.kernel.org, keescook@...omium.org, hpa@...or.com,
	gnomes@...rguk.ukuu.org.uk,
	Matthew Garrett <matthew.garrett@...ula.com>
Subject: [PATCH 10/12] x86: Restrict MSR access when trusted_kernel is true

Permitting write access to MSRs allows userspace to modify the running
kernel. Prevent this if trusted_kernel is true. Based on a patch by Kees
Cook.

Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Matthew Garrett <matthew.garrett@...ula.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 113e707..dec5550 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/processor.h>
 #include <asm/msr.h>
@@ -105,6 +106,9 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
 	int err = 0;
 	ssize_t bytes = 0;
 
+	if (get_trusted_kernel())
+		return -EPERM;
+
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
 
@@ -152,6 +156,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 			err = -EBADF;
 			break;
 		}
+		if (get_trusted_kernel()) {
+			err = -EPERM;
+			break;
+		}
 		if (copy_from_user(&regs, uregs, sizeof regs)) {
 			err = -EFAULT;
 			break;
-- 
2.1.0

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