Date:   Fri, 27 Jan 2023 08:52:09 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        jasowang@redhat.com, virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, elena.reshetova@intel.com,
        kirill.shutemov@linux.intel.com, Andi Kleen <ak@linux.intel.com>,
        Amit Shah <amit@kernel.org>, Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v1 2/6] virtio console: Harden port adding
Message-ID: <20230127085006-mutt-send-email-mst@kernel.org>
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On Fri, Jan 27, 2023 at 02:47:55PM +0200, Alexander Shishkin wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Fri, Jan 27, 2023 at 01:55:43PM +0200, Alexander Shishkin wrote:
> >> "Michael S. Tsirkin" <mst@redhat.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?
> 
> So, for example in TDX, you have certain pages as "shared", as in
> between guest and hypervisor. You can have virtio ring(s) in such
> pages.

That one's marked as dma coherent.

> It's likely that there'd be a swiotlb buffer there instead, but
> sharing pages between host virtio and guest virtio drivers is possible.

It's not something console does though, does it?

> Apologies if the language is confusing, I hope I'm answering the
> question.
> 
> Regards,
> --
> Alex

I'd like an answer to when does the console driver share the buffer
in question, not when generally some pages shared.

-- 
MST