Re: [RFC PATCH v2 0/5] physmem: Have flaview API check bus permission from MemTxAttrs argument

[Peter Xu]

On Mon, Aug 23, 2021 at 08:10:50PM +0100, Peter Maydell wrote:
> On Mon, 23 Aug 2021 at 17:42, Philippe Mathieu-Daudé <philmd@redhat.com> 
> wrote:
> >
> > This series aim to kill a recent class of bug, the infamous
> > "DMA reentrancy" issues found by Alexander while fuzzing.
> >
> > Introduce the 'bus_perm' field in MemTxAttrs, defining 3 bits:
> >
> > - MEMTXPERM_UNSPECIFIED (current default, unchanged behavior)
> > - MEMTXPERM_UNRESTRICTED (allow list approach)
> > - MEMTXPERM_RAM_DEVICE (example of deny list approach)
> >
> > If a transaction permission is not allowed (for example access
> > to non-RAM device), we return the specific MEMTX_BUS_ERROR.
> >
> > Permissions are checked in after the flatview is resolved, and
> > before the access is done, in a new function: flatview_access_allowed().
> 
> So I'm not going to say 'no' to this, because we have a real
> recursive-device-handling problem and I don't have a better
> idea to hand, but the thing about this is that we end up with
> behaviour which is not what the real hardware does. I'm not
> aware of any DMA device which has this kind of "can only DMA
> to/from RAM, and aborts on access to a device" behaviour...

Sorry for not being familiar with the context - is there more info regarding
the problem to fix?  I'm looking at the links mentioned in the old series:

https://lore.kernel.org/qemu-devel/20200903110831.353476-12-philmd@redhat.com/
https://bugs.launchpad.net/qemu/+bug/1886362
https://bugs.launchpad.net/qemu/+bug/1888606

They seem all marked as fixed now.

Thanks,

-- 
Peter Xu