[PATCH V31 12/25] x86/msr: Restrict MSR access when the kernel is locked down

[Matthew Garrett]

From: Matthew Garrett <mjg59 at srcf.ucam.org>

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.

Signed-off-by: Matthew Garrett <mjg59 at google.com>
Signed-off-by: David Howells <dhowells at redhat.com>
Acked-by: Kees Cook <keescook at chromium.org>
Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
cc: x86 at kernel.org
---
 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 4588414e2561..731be1be52b6 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -84,6 +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("Direct MSR access", LOCKDOWN_INTEGRITY))
+		return -EPERM;
+
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
 
@@ -135,6 +138,11 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 			err = -EFAULT;
 			break;
 		}
+		if (kernel_is_locked_down("Direct MSR access",
+					  LOCKDOWN_INTEGRITY)) {
+			err = -EPERM;
+			break;
+		}
 		err = wrmsr_safe_regs_on_cpu(cpu, regs);
 		if (err)
 			break;
-- 
2.21.0.392.gf8f6787159e-goog