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

[Matthew Garrett]

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 <keesc...@chromium.org>
Signed-off-by: Matthew Garrett <matthew.garr...@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

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