Re: [PATCH v1 4/6] virtio console: Harden control message handling

Alexander Shishkin
2023-01-20T16:41:42Z

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Thu, Jan 19, 2023 at 04:22:09PM +0100, Greg Kroah-Hartman wrote:
>> On Thu, Jan 19, 2023 at 03:57:19PM +0200, Alexander Shishkin wrote:
>> > In handle_control_message(), we look at the ->event field twice, which
>> > gives a malicious VMM a window in which to switch it from PORT_ADD to
>> > PORT_REMOVE, triggering a null dereference further down the line:
>>
>> How is the other VMM have full control over the full message here?
>> Shouldn't this all have been copied into our local memory if we are
>> going to be poking around in it? Like I mentioned in my other review,
>> copy it all once and then parse it. Don't try to mess with individual
>> fields one at a time otherwise that way lies madness...
>>
>> thanks,
>>
>> greg k-h
>
> I agree and in fact, it is *already* copied since with malicious
> device we generally use a bounce buffer.

Right, but the code should probably be able to handle bad input on its
own, or what do you think?

> Having said that, the patch is actually a cleanup, e.g. it's clearer
> to byte-swap only once.
> Just don't oversell it as a security thing.

Well, security was the original motivation, so that's what it said in
the commit message. But we settled on [0] yesterday with Greg, which
would replace this patch and 2/6.

[0] https://lore.kernel.org/all/87a62eqo4h.fsf@ubik.fi.intel.com/

Regards,
--
Alex