linux-kernel - Re: [PATCH v1 2/6] virtio console: Harden port adding

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Message-ID: <20230127071152-mutt-send-email-mst@kernel.org>
Date:   Fri, 27 Jan 2023 07:12:52 -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, Andi Kleen <ak@...ux.intel.com>,
        Amit Shah <amit@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v1 2/6] virtio console: Harden port adding

On Fri, Jan 27, 2023 at 01:55:43PM +0200, Alexander Shishkin wrote:
> "Michael S. Tsirkin" <mst@...hat.com> writes:
> 
> > On Thu, Jan 19, 2023 at 10:13:18PM +0200, Alexander Shishkin wrote:
> >> When handling control messages, instead of peeking at the device memory
> >> to obtain bits of the control structure,
> >
> > Except the message makes it seem that we are getting data from
> > device memory, when we do nothing of the kind.
> 
> We can be, see below.
> 
> >> take a snapshot of it once and
> >> use it instead, to prevent it from changing under us. This avoids races
> >> between port id validation and control event decoding, which can lead
> >> to, for example, a NULL dereference in port removal of a nonexistent
> >> port.
> >> 
> >> The control structure is small enough (8 bytes) that it can be cached
> >> directly on the stack.
> >
> > I still have no real idea why we want a copy here.
> > If device can poke anywhere at memory then it can crash kernel anyway.
> > If there's a bounce buffer or an iommu or some other protection
> > in place, then this memory can no longer change by the time
> > we look at it.
> 
> We can have shared pages between the host and guest without bounce
> buffers in between, so they can be both looking directly at the same
> page.
> 
> Regards,

How does this configuration work? What else is in this page?

> --
> Alex