Re: [PATCH] Kernel Lockdown: Add an option to allow raw MSR access even, in confidentiality mode.

[Kees Cook <keescook@chromium.org>]

On Sat, Nov 30, 2019 at 12:49:48AM -0600, Matt Parnell wrote:
> From 452b8460e464422d268659a8abb93353a182f8c8 Mon Sep 17 00:00:00 2001
> From: Matt Parnell <mparnell@gmail.com>
> Date: Sat, 30 Nov 2019 00:44:09 -0600
> Subject: [PATCH] Kernel Lockdown: Add an option to allow raw MSR access even
>  in confidentiality mode.
> 
> For Intel CPUs, some of the MDS mitigations utilize the new "flush" MSR, and
> while this isn't something normally used in userspace, it does cause false
> positives for the "Forshadow" vulnerability.
> 
> Additionally, Intel CPUs use MSRs for voltage and frequency controls,
> which in
> many cases is useful for undervolting to avoid excess heat.
> 
> Signed-off-by: Matt Parnell <mparnell@gmail.com>

I would expect this to just be implemented via LSM policy, not ifdefs
and Kconfig?

-Kees

> ---
>  arch/x86/kernel/msr.c     |  5 ++++-
>  security/lockdown/Kconfig | 12 ++++++++++++
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
> index 1547be359d7f..4adce59455c3 100644
> --- a/arch/x86/kernel/msr.c
> +++ b/arch/x86/kernel/msr.c
> @@ -80,10 +80,11 @@ static ssize_t msr_write(struct file *file, const
> char __user *buf,
>      int err = 0;
>      ssize_t bytes = 0;
>  
> +#if defined(LOCK_DOWN_DENY_RAW_MSR)
>      err = security_locked_down(LOCKDOWN_MSR);
>      if (err)
>          return err;
> -
> +#endif
>      if (count % 8)
>          return -EINVAL;    /* Invalid chunk size */
>  
> @@ -135,9 +136,11 @@ static long msr_ioctl(struct file *file, unsigned
> int ioc, unsigned long arg)
>              err = -EFAULT;
>              break;
>          }
> +#if defined(LOCK_DOWN_DENY_RAW_MSR)
>          err = security_locked_down(LOCKDOWN_MSR);
>          if (err)
>              break;
> +#endif
>          err = wrmsr_safe_regs_on_cpu(cpu, regs);
>          if (err)
>              break;
> diff --git a/security/lockdown/Kconfig b/security/lockdown/Kconfig
> index e84ddf484010..f4fe72c4bf8f 100644
> --- a/security/lockdown/Kconfig
> +++ b/security/lockdown/Kconfig
> @@ -44,4 +44,16 @@ config LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY
>       code to read confidential material held inside the kernel are
>       disabled.
>  
> +config LOCK_DOWN_DENY_RAW_MSR
> +    bool "Lock down and deny raw MSR access"
> +    depends on LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY
> +    default y
> +    help
> +      Some Intel based systems require raw MSR access to use the flush
> +      MSR for MDS mitigation confirmation. Raw access can also be used
> +      to undervolt many Intel CPUs.
> +
> +      Say Y to prevent access or N to allow raw MSR access for such
> +      cases.
> +
>  endchoice
> -- 
> 2.24.0
> 
> 




-- 
Kees Cook