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Subject: Re: [virtio-dev] [PATCH] virtio-iommu: Rework the bypass feature
Hi Jean, On 9/30/21 6:35 PM, Jean-Philippe Brucker wrote: > The VIRTIO_IOMMU_F_BYPASS feature is awkward to use and incomplete. > Although it is implemented by QEMU, it is not supported by any driver as > far as I know. Replace it with a new VIRTIO_IOMMU_F_BYPASS_CONFIG > feature. The old feature bit is deprecated and will be recycled once > versions of QEMU that implement it are not distributed anymore. > > Two features are missing from virtio-iommu: > > * The ability for an hypervisor to start the device in bypass mode. The > wording for VIRTIO_IOMMU_F_BYPASS is not clear enough to allow it at > the moment. > > * The ability for a guest to set individual endpoints in bypass mode > when bypass is globally disabled. An OS should have the ability to > allow only endpoints it trusts to bypass the IOMMU, while keeping DMA > disabled for endpoints it isn't even aware of. At the moment this can > only be emulated by creating identity mappings. > > The VIRTIO_IOMMU_F_BYPASS_CONFIG feature adds a 'bypass' config field > that allows to enable and disable bypass globally. It also adds a new > flag for the ATTACH request. > > * The hypervisor can start the VM with bypass enabled or, if it knows > that the software stack supports it, disabled. The 'bypass' config > fields resets to 0 or 1. I don't see the reset value documented in the actual spec while it looks quite important for our boot bypass functionality. Or did I miss it? > * Generally the firmware won't have an IOMMU driver and will need to be > started in bypass mode, so the bootloader and kernel can be loaded > from storage endpoint. > > For more security, the firmware could implement a minimal virtio-iommu > driver that reuses existing virtio support and only touches the config > space. It could enable PCI bus mastering in bridges only for the > endpoints that need it, enable global IOMMU bypass by flipping a bit, > then tear everything down before handing control over to the OS. This > prevents vulnerability windows where a malicious endpoint reprograms > the IOMMU while the OS is configuring it [1]. > > The isolation provided by vIOMMUs has mainly been used for securely > assigning endpoints to untrusted applications so far, while kernel DMA > bypasses the IOMMU. But we can expect boot security to become as > important in virtualization as it presently is on bare-metal systems, > where some devices are untrusted and must never be able to access > memory that wasn't assigned to them. > > * The OS can enable and disable bypass globally. It can then enable > bypass for individual endpoints by attaching them to bypass domains, > using the new VIRTIO_IOMMU_ATTACH_F_BYPASS flag. It can disable bypass > by attaching them to normal domains. > > [1] IOMMU protection against I/O attacks: a vulnerability and a proof of concept > Morgan, B., Alata, Ã., Nicomette, V. et al. > https://link.springer.com/article/10.1186/s13173-017-0066-7 > > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org> > --- > The virtio-iommu spec with colored diff is available at > https://jpbrucker.net/virtio-iommu/spec-bypass/virtio-iommu-f-bypass-config-v1-diff.pdf > > Apologies for the poorly thought out VIRTIO_IOMMU_F_BYPASS. I didn't > spend enough time on it and ignored valuable suggestions. > --- > virtio-iommu.tex | 69 +++++++++++++++++++++++++++++++++++++----------- > 1 file changed, 53 insertions(+), 16 deletions(-) > > diff --git a/virtio-iommu.tex b/virtio-iommu.tex > index efa735b..a2c90ea 100644 > --- a/virtio-iommu.tex > +++ b/virtio-iommu.tex > @@ -59,14 +59,19 @@ \subsection{Feature bits}\label{sec:Device Types / IOMMU Device / Feature bits} > VIRTIO_IOMMU_F_MAP_UNMAP is supported.} > > \item[VIRTIO_IOMMU_F_BYPASS (3)] > - When not attached to a domain, endpoints downstream of the IOMMU > - can access the guest-physical address space. > + This feature is deprecated. Why are you obliged to deprecate the feature. Can't you leave it and impose that either VIRTIO_IOMMU_F_BYPASS or VIRTIO_IOMMU_F_BYPASS_CONFIG gets set? > > \item[VIRTIO_IOMMU_F_PROBE (4)] > The PROBE request is available. > > \item[VIRTIO_IOMMU_F_MMIO (5)] > The VIRTIO_IOMMU_MAP_F_MMIO flag is available. > + > +\item[VIRTIO_IOMMU_F_BYPASS_CONFIG (6)] > + The global bypass state is described in \field{bypass} of > + struct virtio_iommu_config. The domain bypass state is > + described by VIRTIO_IOMMU_ATTACH_F_BYPASS. I would re-emphasize the global bypass state relates to bypass state of unattached devices whereas domain bypass state controls bypass for devices attached to a given domain. > + > \end{description} > > \drivernormative{\subsubsection}{Feature bits}{Device Types / IOMMU Device / Feature bits} > @@ -97,12 +102,19 @@ \subsection{Device configuration layout}\label{sec:Device Types / IOMMU Device / > le32 end; > } domain_range; > le32 probe_size; > + u8 bypass; > + u8 reserved[3]; > }; > \end{lstlisting} > > \drivernormative{\subsubsection}{Device configuration layout}{Device Types / IOMMU Device / Device configuration layout} > > -The driver MUST NOT write to device configuration fields. > +When the VIRTIO_IOMMU_F_BYPASS_CONFIG feature is negotiated, the > +driver MAY write to \field{bypass}. The driver MUST NOT write to > +any other device configuration field. > + > +If field \field{bypass} contains a value different than 0 or 1, > +the driver SHOULD treat it as 0. > > \devicenormative{\subsubsection}{Device configuration layout}{Device Types / IOMMU Device / Device configuration layout} > > @@ -110,16 +122,24 @@ \subsection{Device configuration layout}\label{sec:Device Types / IOMMU Device / > the page granularity. The device MAY set more than one bit in > \field{page_size_mask}. > > +If the driver writes a value different than 0 or 1 in > +\field{bypass}, the device SHOULD treat it as 0. > + > \subsection{Device initialization}\label{sec:Device Types / IOMMU Device / Device initialization} > > When the device is reset, endpoints are not attached to any domain. > > -If the VIRTIO_IOMMU_F_BYPASS feature is negotiated, all accesses from > -unattached endpoints are allowed and translated by the IOMMU using the > -identity function. If the feature is not negotiated, any memory access > -from an unattached endpoint fails. Upon attaching an endpoint in > -bypass mode to a new domain, any memory access from the endpoint fails, > -since the domain does not contain any mapping. > +An endpoint is in bypass mode if the VIRTIO_IOMMU_F_BYPASS_CONFIG > +feature is offered and: > +\begin{itemize} > + \item config field \field{bypass} is 1 and the endpoint is not > + attached to a domain, or > + \item the endpoint is attached to a domain with > + VIRTIO_IOMMU_ATTACH_F_BYPASS. > +\end{itemize} > + > +All accesses from an endpoint in bypass mode are allowed and > +translated by the IOMMU using the identity function. > > Future devices might support more modes of operation besides MAP/UNMAP. > Drivers verify that devices set VIRTIO_IOMMU_F_MAP_UNMAP and fail > @@ -136,8 +156,8 @@ \subsection{Device initialization}\label{sec:Device Types / IOMMU Device / Devic > > \devicenormative{\subsubsection}{Device Initialization}{Device Types / IOMMU Device / Device Initialization} > > -If the driver does not accept the VIRTIO_IOMMU_F_BYPASS feature, the > -device SHOULD NOT let endpoints access the guest-physical address space. > +The device SHOULD NOT let unattached endpoints that are not in > +bypass mode access the guest-physical address space. SHOULD NOT or MUST NOT. Can you remind me of the difference? > > \subsection{Device operations}\label{sec:Device Types / IOMMU Device / Device operations} > > @@ -266,9 +286,12 @@ \subsubsection{ATTACH request}\label{sec:Device Types / IOMMU Device / Device op > struct virtio_iommu_req_head head; > le32 domain; > le32 endpoint; > - u8 reserved[8]; > + le32 flags; > + u8 reserved[4]; > struct virtio_iommu_req_tail tail; > }; > + > +#define VIRTIO_IOMMU_ATTACH_F_BYPASS (1 << 0) > \end{lstlisting} > > Attach an endpoint to a domain. \field{domain} uniquely identifies a > @@ -294,6 +317,10 @@ \subsubsection{ATTACH request}\label{sec:Device Types / IOMMU Device / Device op > attached to a single domain at a time. Endpoints attached to different > domains are isolated from each other. > > +When the VIRTIO_IOMMU_F_BYPASS_CONFIG is negotiated, the driver > +can set the VIRTIO_IOMMU_ATTACH_F_BYPASS flag to create a bypass > +domain. Endpoints attached to this domain are in bypass mode. > + > \drivernormative{\paragraph}{ATTACH request}{Device Types / IOMMU Device / Device operations / ATTACH request} > > The driver SHOULD set \field{reserved} to zero. > @@ -301,11 +328,20 @@ \subsubsection{ATTACH request}\label{sec:Device Types / IOMMU Device / Device op > The driver SHOULD ensure that endpoints that cannot be isolated from each > other are attached to the same domain. > > +If the domain already exists and is a bypass domain, the driver > +SHOULD set the VIRTIO_IOMMU_ATTACH_F_BYPASS flag. If the domain > +exists and is not a bypass domain, the driver SHOULD NOT set the > +VIRTIO_IOMMU_ATTACH_F_BYPASS flag. > + > \devicenormative{\paragraph}{ATTACH request}{Device Types / IOMMU Device / Device operations / ATTACH request} > > If the \field{reserved} field of an ATTACH request is not zero, the device > MUST reject the request and set \field{status} to VIRTIO_IOMMU_S_INVAL. > > +If the device does not recognize a \field{flags} bit, it MUST > +reject the request and set \field{status} to > +VIRTIO_IOMMU_S_INVAL. > + > If the endpoint identified by \field{endpoint} doesn't exist, the device > MUST reject the request and set \field{status} to VIRTIO_IOMMU_S_NOENT. > > @@ -343,10 +379,7 @@ \subsubsection{DETACH request} > \end{lstlisting} > > Detach an endpoint from a domain. When this request completes, the > -endpoint cannot access any mapping from that domain anymore. If feature > -VIRTIO_IOMMU_F_BYPASS has been negotiated, then once this request > -completes all accesses from the endpoint are allowed and translated by the > -IOMMU using the identity function. > +endpoint cannot access any mapping from that domain anymore. You could adapt the rest of the paragraph to the new VIRTIO_IOMMU_F_BYPASS_CONFIG > > After all endpoints have been successfully detached from a domain, it > ceases to exist and its ID can be reused by the driver for another domain. > @@ -433,6 +466,8 @@ \subsubsection{MAP request}\label{sec:Device Types / IOMMU Device / Device opera > > The driver SHOULD set undefined \field{flags} bits to zero. > > +The driver SHOULD NOT send MAP requests on a bypass domain. > + > \field{virt_end} MUST be strictly greater than \field{virt_start}. > > The driver SHOULD set the VIRTIO_IOMMU_MAP_F_MMIO flag when the physical > @@ -538,6 +573,8 @@ \subsubsection{UNMAP request}\label{sec:Device Types / IOMMU Device / Device ope > or be outside any mapping. The last address of a range MUST either be the > last address of a mapping or be outside any mapping. > > +The driver SHOULD NOT send UNMAP requests on a bypass domain. > + > \devicenormative{\paragraph}{UNMAP request}{Device Types / IOMMU Device / Device operations / UNMAP request} > > If the \field{reserved} field of an UNMAP request is not zero, the device Shouldn't we descrive the behavior of the device is MAP/UNMAP is called on a bypass domain or if we try to attach another device to a domain with a different ATTACH_F_BYPASS flag? Thanks Eric
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