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

From: Matthew Garrett
Date: Tue Mar 26 2019 - 14:28:31 EST


From: Matthew Garrett <mjg59@xxxxxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
cc: x86@xxxxxxxxxx
---
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