[PATCH 10/12] x86: Restrict MSR access when securelevel is set

[Matthew Garrett <matthew.garrett@nebula.com>]

Permitting write access to MSRs allows userspace to modify the running
kernel. Prevent this if securelevel has been set. Based on a patch by Kees
Cook.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.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 88458fa..6b6cb4f 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -37,6 +37,7 @@
 #include <linux/notifier.h>
 #include <linux/uaccess.h>
 #include <linux/gfp.h>
+#include <linux/security.h>
 
 #include <asm/processor.h>
 #include <asm/msr.h>
@@ -103,6 +104,9 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
 	int err = 0;
 	ssize_t bytes = 0;
 
+	if (get_securelevel() > 0)
+		return -EPERM;
+
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
 
@@ -150,6 +154,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 			err = -EBADF;
 			break;
 		}
+		if (get_securelevel() > 0) {
+			err = -EPERM;
+			break;
+		}
 		if (copy_from_user(&regs, uregs, sizeof regs)) {
 			err = -EFAULT;
 			break;
-- 
1.8.3.1