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

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Date:   Fri, 20 Jan 2023 07:45:05 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        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>
Subject: Re: [PATCH v1 4/6] virtio console: Harden control message handling

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.
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.


-- 
MST