[13/27] x86/msr: Restrict MSR access when the kernel is locked down

Commit Message

Matthew Garrett March 6, 2019, 11:58 p.m. UTC

From: Matthew Garrett <[e-mail]>

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 <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Reviewed-by: Thomas Gleixner <[e-mail]>
Reviewed-by: "Lee, Chun-Yi" <[e-mail]>
cc: [e-mail]
Signed-off-by: Matthew Garrett <[e-mail]>
---
 arch/x86/kernel/msr.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Patch

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 4588414e2561..f5a2cf07972f 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;