[PATCH 11/24] x86/msr: Restrict MSR access when the kernel is locked down

From: David Howells
Date: Wed Apr 11 2018 - 12:25:59 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.

MSR accesses are logged for the purposes of building up a whitelist as per
Alan Cox's suggestion.

Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: "Lee, Chun-Yi" <jlee@xxxxxxxx>
cc: x86@xxxxxxxxxx
---

arch/x86/kernel/msr.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index ef688804f80d..dfb61d358196 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -84,6 +84,11 @@ 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")) {
+ pr_info("Direct access to MSR %x\n", reg);
+ return -EPERM;
+ }
+
if (count % 8)
return -EINVAL; /* Invalid chunk size */

@@ -135,6 +140,11 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
err = -EFAULT;
break;
}
+ if (kernel_is_locked_down("Direct MSR access")) {
+ pr_info("Direct access to MSR %x\n", regs[1]); /* Display %ecx */
+ err = -EPERM;
+ break;
+ }
err = wrmsr_safe_regs_on_cpu(cpu, regs);
if (err)
break;