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

[Matt Parnell <mparnell@gmail.com>]

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@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
>>
>>
>
>
>