[PATCH V33 14/30] x86/msr: Restrict MSR access when the kernel is locked down

[Matthew Garrett]

From: Matthew Garrett <mj...@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 <mj...@google.com>
Signed-off-by: David Howells <dhowe...@redhat.com>
Acked-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: Thomas Gleixner <t...@linutronix.de>
cc: x...@kernel.org
---
 arch/x86/kernel/msr.c        | 8 ++++++++
 include/linux/security.h     | 1 +
 security/lockdown/lockdown.c | 1 +
 3 files changed, 10 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 4588414e2561..72f0ed5a93c3 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -39,6 +39,7 @@
 #include <linux/notifier.h>
 #include <linux/uaccess.h>
 #include <linux/gfp.h>
+#include <linux/security.h>
 
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
@@ -84,6 +85,9 @@ static ssize_t msr_write(struct file *file, const char __user 
*buf,
        int err = 0;
        ssize_t bytes = 0;
 
+       if (security_is_locked_down(LOCKDOWN_MSR))
+               return -EPERM;
+
        if (count % 8)
                return -EINVAL; /* Invalid chunk size */
 
@@ -135,6 +139,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, 
unsigned long arg)
                        err = -EFAULT;
                        break;
                }
+               if (security_is_locked_down(LOCKDOWN_MSR)) {
+                       err = -EPERM;
+                       break;
+               }
                err = wrmsr_safe_regs_on_cpu(cpu, regs);
                if (err)
                        break;
diff --git a/include/linux/security.h b/include/linux/security.h
index 59f0ac7adfa6..81c0968e485f 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -88,6 +88,7 @@ enum lockdown_reason {
        LOCKDOWN_HIBERNATION,
        LOCKDOWN_PCI_ACCESS,
        LOCKDOWN_IOPORT,
+       LOCKDOWN_MSR,
        LOCKDOWN_INTEGRITY_MAX,
        LOCKDOWN_CONFIDENTIALITY_MAX,
 };
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 6e426887bb23..a01301972290 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -24,6 +24,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] 
= {
        [LOCKDOWN_HIBERNATION] = "hibernation",
        [LOCKDOWN_PCI_ACCESS] = "direct PCI access",
        [LOCKDOWN_IOPORT] = "raw io port access",
+       [LOCKDOWN_MSR] = "raw MSR access",
        [LOCKDOWN_INTEGRITY_MAX] = "integrity",
        [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };
-- 
2.22.0.410.gd8fdbe21b5-goog