linux-kernel - [PATCH 13/30] x86/msr: Restrict MSR access when the kernel is locked down

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Message-Id: <151024873510.28329.5219612113295367113.stgit@warthog.procyon.org.uk>
Date:   Thu,  9 Nov 2017 17:32:15 +0000
From:   David Howells <dhowells@...hat.com>
To:     linux-security-module@...r.kernel.org
Cc:     gnomes@...rguk.ukuu.org.uk, linux-efi@...r.kernel.org,
        dhowells@...hat.com, linux-kernel@...r.kernel.org,
        jforbes@...hat.com
Subject: [PATCH 13/30] x86/msr: Restrict MSR access when the kernel is locked down

From: Matthew Garrett <matthew.garrett@...ula.com>

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 <matthew.garrett@...ula.com>
Signed-off-by: David Howells <dhowells@...hat.com>
Acked-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: "Lee, Chun-Yi" <jlee@...e.com>
cc: x86@...nel.org
---

 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;