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Subject[PATCH 12/27] x86/msr: Restrict MSR access when the kernel is locked down
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From: Matthew Garrett <matthew.garrett@nebula.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.

Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
cc: x86@kernel.org
---

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 ef688804f80d..a05a97863286 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -84,6 +84,9 @@ 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"))
+ return -EPERM;
+
if (count % 8)
return -EINVAL; /* Invalid chunk size */

@@ -131,6 +134,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
err = -EBADF;
break;
}
+ if (kernel_is_locked_down("Direct MSR access")) {
+ err = -EPERM;
+ break;
+ }
if (copy_from_user(&regs, uregs, sizeof regs)) {
err = -EFAULT;
break;
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