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Re: [PATCH] x86/amd: Drop allow_unsafe parameter, tune down XSA-9 mitigations
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 4 May 2026 12:35:30 +0200
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- Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 04 May 2026 10:35:41 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24.04.2026 11:18, Andrew Cooper wrote:
> On 22/04/2026 5:58 pm, Teddy Astie wrote:
>> XSA-9 mitigations prevents Xen from properly running if a affected CPU
>> is detected. While the vulnerability has no mitigations (aside not running
>> 64-bits PV guests), it's only outcome is a DoS.
>>
>> There is no real point in preventing Xen from working here and it would
>> be preferable to just log the vulnerability to the user so it can act
>> appropriately.
>>
>> Also reword the errata message regarding that it only affects PV64 guests.
>>
>> Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
>> ---
>> It's hard to tell whether or not we would want to drop allow_unsafe, but
>> currently, Xen is allowed to boot with XSA-304 mitigations disabled
>> (intentionnaly) which has a similar outcome than this one.
>>
>> From a user standpoint, preventing the system to boot or starting guests is
>> too extreme and would be in practice seen as a bug, even if it's justified
>> policy which can be overriden.
>
> There is a lot of history here, including for why the panic() is
> actually completely unreachable.
Deliberately so, as described in 0c7a69665110: The change done allows for
a trivial 1-line change restoring prior default behavior (in distros which
may want this).
> I actually have a very similar patch, from almost certainly more than a
> decade ago. The difference is that now I'm a maintainer and in a better
> position to say that this logic really shouldn't have been taken;
> certainly not in this form.
>
> Perhaps an easier justification is that there have been issues like AMD
> erratum #121 prior (e.g. Pentium LOCK CMPXCHG) and post (Zen1 has a
> related errata, can't remember it off the top of my head) that went
> unaddressed.
I agree it would be better if we were consistent in treatment of errata
(somewhat) similar in effect. I'm less sure which direction we should go
in order to achieve consistency.
Jan
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