Re: [PATCH v2 1/3] memory: Track whether a Device is engaged in IO

[Peter Maydell]

On Fri, 27 May 2022 at 17:19, Alexander Bulekov <alxndr@bu.edu> wrote:
>
> Add a flag to the DeviceState, when a device is engaged in PIO/MMIO/DMA.
> This flag should be set/checked prior to calling a device's MemoryRegion
> handlers, and set when device code initiates DMA.  The purpose of this
> flag is to prevent DMA reentrancy issues. E.g.:
> sdhci pio -> dma write -> sdhci mmio
> nvme bh -> dma write -> nvme mmio
>
> These issues have led to problems such as stack-exhaustion and
> use-after-frees.
>
> Assumptions:
>  * Devices do not interact with their own PIO/MMIO memory-regions using
>    DMA.

If you're trying to protect against malicious guest-controlled
DMA operations, you can't assume that. The guest can program
a DMA controller to DMA to its own MMIO register bank if it likes.

>  * There is now way for there to be multiple simultaneous accesses to a
>    device's PIO/MMIO memory-regions, or for multiple threads to perform
>    DMA accesses simultaneously on behalf of a single device.

This one is generally true because device code runs with
the iothread lock held.

-- PMM