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Subject: Re: [PATCH 2/2] drivers/misc: Add Aspeed P2A control driver
To:     Patrick Venture <venture@google.com>, arnd@arndb.de,
        gregkh@linuxfoundation.org, joel@jms.id.au, andrew@aj.id.au
Cc:     linux-aspeed@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
References: <20190221222537.137331-1-venture@google.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
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Message-ID: <9ab3a0e6-fa1e-11c7-adad-7e14bfe28059@gmail.com>
Date:   Tue, 26 Feb 2019 21:04:52 -0800
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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?

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.
-- 
Florian