linux-kernel - Re: [PATCH 0/2] virt: vmgenid: add generation counter

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Message-ID: <14961f48-93d9-1193-b4c4-bcc232016b7d@amazon.es>
Date:   Thu, 4 Aug 2022 17:46:30 +0200
From:   <bchalios@...zon.es>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        <linux-kernel@...r.kernel.org>, <tytso@....edu>,
        <dwmw@...zon.co.uk>, <graf@...zon.de>, <xmarcalx@...zon.co.uk>,
        <gregkh@...uxfoundation.org>, <mikelley@...rosoft.com>
Subject: Re: [PATCH 0/2] virt: vmgenid: add generation counter

Hi Jason,

On 8/4/22 4:59 PM, "Jason A. Donenfeld" <Jason@...c4.com> wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> Hi Babis,
> 
> On Wed, Aug 03, 2022 at 05:21:25PM +0200, bchalios@...zon.es wrote:
> > From: Babis Chalios <bchalios@...zon.es>
> >
> > Linux recently added support for the VM Generation ID mechanism from
> > Microsoft. The way this works currently is using the 128-bit blob
> > provided by the vmgenid device to re-seed the RNG. While this works it
> > has two main issues, (a) it is inherently racy due to the fact that it
> > relies on a ACPI notification being delivered and handled and (b) the ID
> > is unsuitable for exposing to user-space.
> >
> > This patch-set extends the vmgenid device to introduce a generation
> > counter, a 32-bit counter which is different every time the unique ID
> > changes. The addition to the original implementation in QEMU can be
> > found here:
> > https://lists.nongnu.org/archive/html/qemu-devel/2022-08/msg00524.html.
> >
> > The first patch re-works slightly the current vmgenid driver to add a
> > function that parses an object from the vmgenid device and returns the
> > physical address of the vmgenid data. The second patch uses that
> > function to parse additionally the address of the generation counter
> > from the vmgenid namespace. The counter is then exposed to the
> > user-space through a misc-device which provides `read` and `mmap`
> > interfaces.
> 
> First, with regards to your mmap interface, it's more likely that this
> kind of thing will be eventually folded into my investigations regarding
> the RNG and the vDSO (which would make this kind of thing accessible
> without needing the file system).
> 

Interesting, could you share a link to discussions / code regarding this?


> Regarding the counter itself, I don't want to rush into augmenting the
> vmgenid mechanism until we've had some conversations with Microsoft. But
> also, it seems like you might have missed the extensive previous
> discussion about this. There was some tradeoff in efficiency about
> mapping this all the way through, as doing so would require the counter
> to be in a totally separate page as the main 128-bit ID, versus just
> having the kernel manage a separate counter and incur a potential [maybe
> acceptable? unclear] race.
> 

Again, would you have pointers to this discussion? I have been following this
thread: https://lkml.org/lkml/2022/3/1/693. I am interested on the take that
the race would be acceptable.

> Jason
>

Cheers,
Babis

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