[PATCH] x86: Lock down MSR writing in secure boot

[Kees Cook]

Writing to MSRs should not be allowed unless CAP_COMPROMISE_KERNEL is
set since it could lead to execution of arbitrary code in kernel mode.

Signed-off-by: Kees Cook <keesc...@chromium.org>
---
This would be used on top of Matthew Garrett's existing "Secure boot
policy support" patch series.
---
 arch/x86/kernel/msr.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 4929502..adaab3d 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -103,6 +103,9 @@ static ssize_t msr_write(struct file *file, const char 
__user *buf,
        int err = 0;
        ssize_t bytes = 0;
 
+       if (!capable(CAP_COMPROMISE_KERNEL))
+               return -EPERM;
+
        if (count % 8)
                return -EINVAL; /* Invalid chunk size */
 
@@ -150,6 +153,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, 
unsigned long arg)
                        err = -EBADF;
                        break;
                }
+               if (!capable(CAP_COMPROMISE_KERNEL)) {
+                       err = -EPERM;
+                       break;
+               }
                if (copy_from_user(&regs, uregs, sizeof regs)) {
                        err = -EFAULT;
                        break;
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
--
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