[PATCH 13/27] 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.

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

Signed-off-by: Matthew Garrett <mj...@srcf.ucam.org>
Signed-off-by: David Howells <dhowe...@redhat.com>
Acked-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: Thomas Gleixner <t...@linutronix.de>
Reviewed-by: "Lee, Chun-Yi" <j...@suse.com>
cc: x...@kernel.org
Signed-off-by: Matthew Garrett <matthewgarr...@google.com>
---
 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 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;
-- 
2.21.0.352.gf09ad66450-goog