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

From: Tim Harvey
Date: Fri Oct 04 2019 - 11:23:18 EST

On Thu, Oct 3, 2019 at 3:24 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 2019-10-03 9:51 pm, Tim Harvey wrote:
> > On Thu, Oct 3, 2019 at 1:42 PM Robin Murphy <robin.murphy@xxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxx>
> >>>> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> >>>> ---
> >>>
> >>> 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 */

Robin,

No difference... still need 'arm-smmu.disable_bypass=n' to boot. Are
all four iommu-map props above supposed to be the same? Seems to me
they all point to the same thing which looks wrong.

Tim