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

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Date:   Fri, 20 Jan 2023 18:41:27 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, elena.reshetova@...el.com,
        kirill.shutemov@...ux.intel.com, Amit Shah <amit@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH v1 4/6] virtio console: Harden control message handling

"Michael S. Tsirkin" <mst@...hat.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