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

[Matt Parnell]

That is, I was intending to use lockdown from boot, which isn't
changeable after the fact if I'm not mistaken. How possible is granular
control of what is and is not locked down?

On 11/30/19 1:09 PM, Matt Parnell wrote:
> I can see how using a policy would be beneficial; I only did this
> because as I understood it, policy wouldn't be able to change these
> particular settings since anything attempting to do so would be from
> userspace.
>
> On 11/30/19 12:36 PM, Kees Cook wrote:
>> 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 at 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 at 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
>>>
>>>
>>
>>