Re: [PATCH for 8.0 v3] memory: Prevent recursive memory access

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 16 Mar 2023 at 16:21, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> A guest may request ask a memory-mapped device to perform DMA. If the
> address specified for DMA is the device performing DMA, it will create
> recursion. It is very unlikely that device implementations are prepared
> for such an abnormal access, which can result in unpredictable behavior.
>
> In particular, such a recursion breaks e1000e, a network device. If
> the device is configured to write the received packet to the register
> to trigger receiving, it triggers re-entry to the Rx logic of e1000e.
> This causes use-after-free since the Rx logic is not re-entrant.
>
> As there should be no valid reason to perform recursive memory access,
> check for recursion before accessing memory-mapped device.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1543
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> ---
> V1 -> V2: Marked the variable thread-local. Introduced linked list.

Don't we already have proposals on the mailing list for
addressing this? How does this patch differ from those?

Also, "device A DMAs to device A" is only a subset of the problems --
you can also have "device A DMAs to device B which triggers DMA to
device A" and more complicated situations.

thanks
-- PMM