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

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Message-ID: <20230127055514-mutt-send-email-mst@kernel.org>
Date:   Fri, 27 Jan 2023 05:58:30 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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 Fri, Jan 20, 2023 at 06:41:27PM +0200, Alexander Shishkin wrote:
> "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?

Basically I think it's ok to look at the same field twice unless
it's mapped as dma coherent. Is that what you are asking about?

> > 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,

At this point I will drop this series and pls post new series
with just the stuff you want included. Include acks if patches
are unchanged.

Thanks!

> --
> Alex