Re: [PATCH v2] iommu/arm-smmu: Break insecure users by disabling bypass by default

[Robin Murphy <robin.murphy@arm.com>]

On 2019-10-03 9:51 pm, Tim Harvey wrote:
> On Thu, Oct 3, 2019 at 1:42 PM Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> Hi Tim,
>>
>> On 2019-10-03 7:27 pm, Tim Harvey wrote:
>>> On Fri, Mar 1, 2019 at 11:21 AM Douglas Anderson <dianders@chromium.org> wrote:
>>>>
>>>> If you're bisecting why your peripherals stopped working, it's
>>>> probably this CL.  Specifically if you see this in your dmesg:
>>>>     Unexpected global fault, this could be serious
>>>> ...then it's almost certainly this CL.
>>>>
>>>> Running your IOMMU-enabled peripherals with the IOMMU in bypass mode
>>>> is insecure and effectively disables the protection they provide.
>>>> There are few reasons to allow unmatched stream bypass, and even fewer
>>>> good ones.
>>>>
>>>> This patch starts the transition over to make it much harder to run
>>>> your system insecurely.  Expected steps:
>>>>
>>>> 1. By default disable bypass (so anyone insecure will notice) but make
>>>>      it easy for someone to re-enable bypass with just a KConfig change.
>>>>      That's this patch.
>>>>
>>>> 2. After people have had a little time to come to grips with the fact
>>>>      that they need to set their IOMMUs properly and have had time to
>>>>      dig into how to do this, the KConfig will be eliminated and bypass
>>>>      will simply be disabled.  Folks who are truly upset and still
>>>>      haven't fixed their system can either figure out how to add
>>>>      'arm-smmu.disable_bypass=n' to their command line or revert the
>>>>      patch in their own private kernel.  Of course these folks will be
>>>>      less secure.
>>>>
>>>> Suggested-by: Robin Murphy <robin.murphy@arm.com>
>>>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
>>>> ---
>>>
>>> Hi Doug / Robin,
>>>
>>> I ran into this breaking things on OcteonTx boards based on CN80XX
>>> CPU. The IOMMU configuration is a bit beyond me and I'm hoping you can
>>> offer some advice. The IOMMU here is cavium,smmu-v2 as defined in
>>> https://github.com/Gateworks/dts-newport/blob/master/cn81xx-linux.dtsi
>>>
>>> Booting with 'arm-smmu.disable_bypass=n' does indeed work around the
>>> breakage as the commit suggests.
>>>
>>> Any suggestions for a proper fix?
>>
>> Ah, you're using the old "mmu-masters" binding (and in a way which isn't
>> well-defined - it's never been specified what the stream ID argument(s)
>> would mean for a PCI host bridge, and Linux just ignores them). The
>> ideal thing would be to update the DT to generic "iommu-map" properties
>> - it's been a long time since I last played with a ThunderX, but I
>> believe the SMMU stream IDs should just be the same as the ITS device
>> IDs (which is how the "mmu-masters" mapping would have played out anyway).
>>
>> The arm-smmu driver support for the old binding has always relied on
>> implicit bypass - there are technical reasons why we can't realistically
>> support the full functionality offered to the generic bindings, but it
>> would be possible to add some degree of workaround to prevent it
>> interacting quite so poorly with disable_bypass, if necessary. Do you
>> have deployed systems with DTs that can't be updated, but still might
>> need to run new kernels?
>>
> 
> Robin,
> 
> Thanks for the response. I don't care too much about supporting new
> kernels with the current DT - I'm good with fixing this with a DT
> change. Would you be able to give me an example? I would love to see
> Cavium mainline an cn81xx dts/dtsi in arch/arm64/boot/dts to be used
> as a base as the only thing we have to go off of currently is the
> Cavium SDK which has fairly old kernel support.

No promises (it's a late-night hack from my sofa), but try giving this a 
go...

Robin.

----->8-----
diff --git a/cn81xx-linux.dtsi b/cn81xx-linux.dtsi
index 3b759d9575fe..dabc9047c674 100644
--- a/cn81xx-linux.dtsi
+++ b/cn81xx-linux.dtsi
@@ -234,7 +234,7 @@
  			clocks = <&sclk>;
  		};

-		smmu0@830000000000 {
+		smmu: smmu0@830000000000 {
  			compatible = "cavium,smmu-v2";
  			reg = <0x8300 0x0 0x0 0x2000000>;
  			#global-interrupts = <1>;
@@ -249,23 +249,18 @@
  				     <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>,
  				     <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>,
  				     <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>, <0 69 4>;
-
-			mmu-masters = <&ecam0 0x100>,
-				      <&pem0  0x200>,
-				      <&pem1  0x300>,
-				      <&pem2  0x400>;
-
+			#iommu-cells = <1>;
+			dma-coherent;
  		};

  		ecam0: pci@848000000000 {
  			compatible = "pci-host-ecam-generic";
  			device_type = "pci";
-			msi-parent = <&its>;
  			msi-map = <0 &its 0 0x10000>;
+			iommu-map = <0 &smmu 0 0x10000>;
  			bus-range = <0 31>;
  			#size-cells = <2>;
  			#address-cells = <3>;
-			#stream-id-cells = <1>;
  			u-boot,dm-pre-reloc;
  			dma-coherent;
  			reg = <0x8480 0x00000000 0 0x02000000>;	 /* Configuration space */
@@ -399,12 +394,11 @@

  			compatible = "cavium,pci-host-thunder-pem";
  			device_type = "pci";
-			msi-parent = <&its>;
  			msi-map = <0 &its 0 0x10000>;
+			iommu-map = <0 &smmu 0 0x10000>;
  			bus-range = <0x1f 0x57>;
  			#size-cells = <2>;
  			#address-cells = <3>;
-			#stream-id-cells = <1>;
  			dma-coherent;
  			reg = <0x8800 0x1f000000 0x0 0x39000000>,  /* Configuration space */
  				<0x87e0 0xc0000000 0x0 0x01000000>; /* PEM space */
@@ -424,12 +418,11 @@
  		pem1: pci@87e0c1000000 {
  			compatible = "cavium,pci-host-thunder-pem";
  			device_type = "pci";
-			msi-parent = <&its>;
  			msi-map = <0 &its 0 0x10000>;
+			iommu-map = <0 &smmu 0 0x10000>;
  			bus-range = <0x57 0x8f>;
  			#size-cells = <2>;
  			#address-cells = <3>;
-			#stream-id-cells = <1>;
  			dma-coherent;
  			reg = <0x8840 0x57000000 0x0 0x39000000>,  /* Configuration space */
  				<0x87e0 0xc1000000 0x0 0x01000000>; /* PEM space */
@@ -449,12 +442,11 @@
  		pem2: pci@87e0c2000000 {
  			compatible = "cavium,pci-host-thunder-pem";
  			device_type = "pci";
-			msi-parent = <&its>;
  			msi-map = <0 &its 0 0x10000>;
+			iommu-map = <0 &smmu 0 0x10000>;
  			bus-range = <0x8f 0xc7>;
  			#size-cells = <2>;
  			#address-cells = <3>;
-			#stream-id-cells = <1>;
  			dma-coherent;
  			reg = <0x8880 0x8f000000 0x0 0x39000000>,  /* Configuration space */
  				<0x87e0 0xc2000000 0x0 0x01000000>; /* PEM space */