Re: [PATCH] intel_irq_remapping: Clean up x2apic optout security warning mess

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2013-02-01 at 14:57 -0800, Andy Lutomirski wrote:
> Current kernels print this on my Dell server:
> 
>    ------------[ cut here ]------------
>    WARNING: at drivers/iommu/intel_irq_remapping.c:542
>    intel_enable_irq_remapping+0x7b/0x27e()
>    Hardware name: PowerEdge R620
>    Your BIOS is broken and requested that x2apic be disabled
>    This will leave your machine vulnerable to irq-injection attacks
>    Use 'intremap=no_x2apic_optout' to override BIOS request
>    [...]
>    Enabled IRQ remapping in xapic mode
>    x2apic not enabled, IRQ remapping is in xapic mode
> 
> This is inconsistent with itself -- interrupt remapping is *on*.
> 
> Fix the mess by making the warnings say what they mean and my making
> sure that compatibility format interrupts (the dangerous ones) are
> disabled if x2apic is present regardless of BIOS settings.
> 
> With this patch applied, the output is:
> 
>   Your BIOS is broken and requested that x2apic be disabled.
>   This will slightly decrease performance.
>   Use 'intremap=no_x2apic_optout' to override BIOS request.
>   Enabled IRQ remapping in xapic mode
>   x2apic not enabled, IRQ remapping is in xapic mode
> 
> This should make us as or more secure than we are now and replace
> a rather scary warning with a much less scary warning on silly
> but functional systems.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  drivers/iommu/intel_irq_remapping.c | 36 ++++++++++++++++++++++++++++--------
>  1 file changed, 28 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/iommu/intel_irq_remapping.c b/drivers/iommu/intel_irq_remapping.c
> index af8904d..eca8832 100644
> --- a/drivers/iommu/intel_irq_remapping.c
> +++ b/drivers/iommu/intel_irq_remapping.c
> @@ -425,11 +425,22 @@ static void iommu_set_irq_remapping(struct intel_iommu *iommu, int mode)
>  
>  	/* Enable interrupt-remapping */
>  	iommu->gcmd |= DMA_GCMD_IRE;
> +	iommu->gcmd &= ~DMA_GCMD_CFI;  /* Block compatibility-format MSIs */
>  	writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
>  
>  	IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
>  		      readl, (sts & DMA_GSTS_IRES), sts);
>  
> +	/*
> +	 * With CFI clear in the Global Command register, we should be
> +	 * protected from dangerous (i.e. compatibility) interrupts
> +	 * regardless of x2apic status.  Check just to be sure.
> +	 */
> +	if (sts & DMA_GSTS_CFIS)
> +		WARN(1, KERN_WARNING
> +			"Compatibility-format IRQs enabled despite intr remapping;\n"
> +			"you are vulnerable to IRQ injection.\n");
> +
>  	raw_spin_unlock_irqrestore(&iommu->register_lock, flags);
>  }
>  
> @@ -526,20 +537,24 @@ static int __init intel_irq_remapping_supported(void)
>  static int __init intel_enable_irq_remapping(void)
>  {
>  	struct dmar_drhd_unit *drhd;
> +	bool x2apic_present;
>  	int setup = 0;
>  	int eim = 0;
>  
> +	x2apic_present = x2apic_supported();
> +
>  	if (parse_ioapics_under_ir() != 1) {
>  		printk(KERN_INFO "Not enable interrupt remapping\n");
> -		return -1;
> +		goto error;
>  	}
>  
> -	if (x2apic_supported()) {
> +	if (x2apic_present) {
>  		eim = !dmar_x2apic_optout();
> -		WARN(!eim, KERN_WARNING
> -			   "Your BIOS is broken and requested that x2apic be disabled\n"
> -			   "This will leave your machine vulnerable to irq-injection attacks\n"
> -			   "Use 'intremap=no_x2apic_optout' to override BIOS request\n");
> +		if (!eim)
> +			printk(KERN_WARNING
> +				"Your BIOS is broken and requested that x2apic be disabled.\n"
> +				"This will slightly decrease performance.\n"
> +				"Use 'intremap=no_x2apic_optout' to override BIOS request.\n");
>  	}
>  
>  	for_each_drhd_unit(drhd) {
> @@ -578,7 +593,7 @@ static int __init intel_enable_irq_remapping(void)
>  		if (eim && !ecap_eim_support(iommu->ecap)) {
>  			printk(KERN_INFO "DRHD %Lx: EIM not supported by DRHD, "
>  			       " ecap %Lx\n", drhd->reg_base_addr, iommu->ecap);
> -			return -1;
> +			goto error;
>  		}
>  	}
>  
> @@ -594,7 +609,7 @@ static int __init intel_enable_irq_remapping(void)
>  			printk(KERN_ERR "DRHD %Lx: failed to enable queued, "
>  			       " invalidation, ecap %Lx, ret %d\n",
>  			       drhd->reg_base_addr, iommu->ecap, ret);
> -			return -1;
> +			goto error;
>  		}
>  	}
>  
> @@ -625,6 +640,11 @@ error:
>  	/*
>  	 * handle error condition gracefully here!
>  	 */
> +
> +	if (x2apic_present)
> +		WARN(1, KERN_WARNING
> +			"Failed to enable irq remapping.  You are vulnerable to irq-injection attacks.\n");

How so?  Without interrupt remapping, neither KVM assignment or vfio
work unless the user explicitly opts-in to insecure interrupts via
module options.  Are there other attack vectors?  Also, I really like
the code above that forces CFI clear, but I don't think that warning is
correct yet.  According to the paper the vulnerability is enabled if
both x2apic is not enabled and CFI is enabled.  When x2apic is enabled,
does it matter if CFI is enabled?  My understand is no.  Should a
warning only occur if the interrupt remapper is enabled and x2apic is
not enabled and we're not able to clear CFI?  Thanks,

Alex