[PATCH] x86: Lock down MSR writing in secure boot

Writing to MSRs should not be allowed unless CAP_COMPROMISE_KERNEL is
set since it could lead to execution of arbitrary code in kernel mode.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
This would be used on top of Matthew Garrett's existing "Secure boot
policy support" patch series.
---
 arch/x86/kernel/msr.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 4929502..adaab3d 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -103,6 +103,9 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
 	int err = 0;
 	ssize_t bytes = 0;
 
+	if (!capable(CAP_COMPROMISE_KERNEL))
+		return -EPERM;
+
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
 
@@ -150,6 +153,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 			err = -EBADF;
 			break;
 		}
+		if (!capable(CAP_COMPROMISE_KERNEL)) {
+			err = -EPERM;
+			break;
+		}
 		if (copy_from_user(&regs, uregs, sizeof regs)) {
 			err = -EFAULT;
 			break;
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security