linux-kernel - Re: [PATCH 14/24] x86: Restrict MSR access when the kernel is locked down

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Date:   Wed, 5 Apr 2017 10:34:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Howells <dhowells@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Matthew Garrett <matthew.garrett@...ula.com>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        keyrings@...r.kernel.org
Subject: Re: [PATCH 14/24] x86: Restrict MSR access when the kernel is locked down

On Wed, Apr 5, 2017 at 10:12 AM, David Howells <dhowells@...hat.com> wrote:
> 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.
>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Matthew Garrett <matthew.garrett@...ula.com>
> Signed-off-by: David Howells <dhowells@...hat.com>

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>
>  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..fbcce028e502 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())
> +               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()) {
> +                       err = -EPERM;
> +                       break;
> +               }
>                 if (copy_from_user(&regs, uregs, sizeof regs)) {
>                         err = -EFAULT;
>                         break;
>



-- 
Kees Cook
Pixel Security