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

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Date:   Fri, 27 Jan 2023 13:55:43 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.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>,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH v1 2/6] virtio console: Harden port adding

"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,
--
Alex