Re: [PATCH -v2.1] x86/msr: Filter MSR writes

[Sean Christopherson]

On Mon, Jun 15, 2020 at 08:38:37AM +0200, Borislav Petkov wrote:
> Here's an improved v2 with sparse warnings fixed:
> 
> ---
> Disable writing to MSRs from userspace by default. Writes can still be
> allowed by supplying the allow_writes=1 module parameter and the kernel
> will be tainted so that it shows in oopses.
> 
> Having unfettered access to all MSRs on a system is and has always been
> a disaster waiting to happen. Think performance counter MSRs, MSRs with
> sticky or locked bits, MSRs making major system changes like loading
> microcode, MTRRs, PAT configuration, TSC counter, security mitigations
> MSRs, you name it.
> 
> This also destroys all the kernel's caching of MSR values for
> performance, as the recent case with MSR_AMD64_LS_CFG showed.
> 
> Another example is writing MSRs by mistake by simply typing the wrong
> MSR address. System freezes have been experienced that way.
> 
> In general, poking at MSRs under the kernel's feet is a bad bad idea.
> 
> So disable poking directly at the MSRs by default. If userspace still
> wants to do that, then proper interfaces should be defined which
> are under the kernel's control and accesses to those MSRs can be
> synchronized and sanitized properly.
> 
> Changelog:
> - taint before WRMSR, all
> - make param 0600, Sean.
> - do not deny but log writes by default, Linus.
> 
> [ Fix sparse warnings ]
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>

A few non-functional nits below.

Tested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

> ---
>  arch/x86/kernel/msr.c | 69 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 69 insertions(+)
> 
> diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
> index 1547be359d7f..576c43e39247 100644
> --- a/arch/x86/kernel/msr.c
> +++ b/arch/x86/kernel/msr.c
> @@ -42,6 +42,14 @@
>  static struct class *msr_class;
>  static enum cpuhp_state cpuhp_msr_state;
>  
> +enum allow_write_msrs {
> +	MSR_WRITES_ON,
> +	MSR_WRITES_OFF,
> +	MSR_WRITES_DEFAULT,
> +};
> +
> +static enum allow_write_msrs allow_writes = MSR_WRITES_DEFAULT;
> +
>  static ssize_t msr_read(struct file *file, char __user *buf,
>  			size_t count, loff_t *ppos)
>  {
> @@ -70,6 +78,24 @@ static ssize_t msr_read(struct file *file, char __user *buf,
>  	return bytes ? bytes : err;
>  }
>  
> +static int filter_write(u32 reg)
> +{
> +     switch (allow_writes) {
> +     case MSR_WRITES_ON:  return 0;          break;
> +     case MSR_WRITES_OFF: return -EPERM;     break;

The breaks after the returns are unnecessary.

> +     default: break;
> +     }
> +
> +     if (reg == MSR_IA32_ENERGY_PERF_BIAS)
> +             return 0;
> +
> +     pr_err_ratelimited("Write to unrecognized MSR 0x%x by %s\n"
> +                        "Please report to x86@xxxxxxxxxx\n",
> +                        reg, current->comm);


Maybe s/unrecognized/unauthorized?  Unrecognized implies the kernel doesn't
know anything about the MSR being written, which may not hold true.

> +     return 0;
> +}
> +
>  static ssize_t msr_write(struct file *file, const char __user *buf,
>                        size_t count, loff_t *ppos)
>  {
> @@ -84,6 +110,10 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
>       if (err)
>               return err;
>
> +     err = filter_write(reg);
> +     if (err)
> +             return err;
> +
>       if (count % 8)
>               return -EINVAL; /* Invalid chunk size */
>
> @@ -92,9 +122,13 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
>                       err = -EFAULT;
>                       break;
>               }
> +
> +             add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> +
>               err = wrmsr_safe_on_cpu(cpu, reg, data[0], data[1]);
>               if (err)
>                       break;
> +

Random leftover whitespace change.

>  		tmp += 2;
>  		bytes += 8;
>  	}