[PATCH 2/2] drivers/misc: Add Aspeed P2A control driver

Patrick Venture venture at google.com
Thu Feb 28 02:17:35 AEDT 2019

On Tue, Feb 26, 2019 at 9:05 PM Florian Fainelli <f.fainelli at gmail.com> wrote:
> On 2/21/2019 2:25 PM, Patrick Venture wrote:
> > The ASPEED AST2400, and AST2500 in some configurations include a
> > PCI-to-AHB MMIO bridge.  This bridge allows a server to read and write
> > in the BMC's memory space.  This feature is especially useful when using
> > this bridge to send large files to the BMC.
> >
> > The host may use this to send down a firmware image by staging data at a
> > specific memory address, and in a coordinated effort with the BMC's
> > software stack and kernel, transmit the bytes.
> >
> > This driver enables the BMC to unlock the PCI bridge on demand, and
> > configure it via ioctl to allow the host to write bytes to an agreed
> > upon location.  In the primary use-case, the region to use is known
> > apriori on the BMC, and the host requests this information.  Once this
> > request is received, the BMC's software stack will enable the bridge and
> > the region and then using some software flow control (possibly via IPMI
> > packets), copy the bytes down.  Once the process is complete, the BMC
> > will disable the bridge and unset any region involved.
> >
> > The default behavior of this bridge when present is: enabled and all
> > regions marked read-write.  This driver will fix the regions to be
> > read-only and then disable the bridge entirely.
> A complete drive by review, so I could be completely off here (most
> likely am), but have you considered using virtio and doing some sort of
> rudimentary features (regions here) negotiation over that interface?

I have not.

> If I get your description right in premise maybe emulating the AHB side
> on the BMC as a PCI end-point device driver, and using it as a seemingly
> regular PCI EP from the host side with BARs and stuff might make sense
> here and be less of a security hole than it currently looks like.

In this case, what I"m trying to do is control access to regions of
BMC physical address space.  The ASPEED BMC is by default, completely
open in this regard, see CVE-2019-6260.  There are two sets of
registers that control the host's ability to read or write the
physical address space.  The host needs to be able to write to the
BMC's physical address space in some use-cases -- one of which is my
firmware staging case.  It could just as easily be used as a memory
buffer region for a virtual nic.  Part of the goal here though is that
the host should not have control of what regions are on/off without
the BMC allowing it.  It's currently a security hole, and this driver
is meant to open that hole on demand for specific purposes, whereas
the default state is completely open.

> --
> Florian

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