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Date: Mon, 25 Sep 2023 20:28:34 +0200
From: Solar Designer <solar@...nwall.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: oss-security@...ts.openwall.com,
"Xen. org security team" <security-team-members@....org>
Subject: Re: Xen Security Advisory 439 v1 (CVE-2023-20588) - x86/AMD: Divide speculative information leak
On Mon, Sep 25, 2023 at 06:10:05PM +0100, Andrew Cooper wrote:
> On 25/09/2023 5:36 pm, Solar Designer wrote:
> > While I am at it, here's the corresponding mitigation in Linux kernel:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=77245f1c3c6495521f6a3af082696ee2f8ce3921
>
> Not really. That patch entirely misunderstood the vulnerability. I
> went through several rounds of getting AMD to better-understand their bug.
>
> Linux's fix was rewritten in
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f58d6fbcb7c848b7f2469be339bc571f2e9d245b
> and this implements the same logic as I implemented in Xen.
Oh wow. Thank you for correcting me (and correcting AMD first?)
> It's worth noting that because AMD did not allocate a $FOO_NO CPUID bit,
> there's no ability for a VM to figure out that it might move to
> vulnerable hardware and therefore should engage the workaround. The
> best a VM can do is best-effort based on whether it looks like it's
> booting on a Zen1 system.
Maybe directly probing for the bug is an option? Perhaps can be done
within one thread (where the bug doesn't have security impact, but is
detectable anyway, no)?
> Also the cross-thread nature is also poorly reported in public.
Right, I couldn't find it mentioned anywhere other than your advisory.
Do you know if only the quotient leaks, or also the remainder? In the
below, I assume the remainder leaks as well.
I'm concerned it could affect some cryptographic code, in particular
(but in a very minor way) typical implementations of Argon2. There's a
3-year-pending pull request to the upstream/reference Argon2
implementation that I think would avoid the issue there (by optimizing
out the divides):
https://github.com/P-H-C/phc-winner-argon2/pull/306
but there are many other implementations and I guess (almost?) all use
the programming language's modulo division operation as-is. Luckily,
the severity is minor - this would only affect the cache-timing unsafe
flavors, providing an extra (more direct and maybe more reliable?)
side-channel, and this only matters when the attacker has a copy of or
has guessed the salts (the same as for other cache-timing unsafe
password hashes/KDFs). So in terms of threat models and attack vectors,
no change at all, but real-world (in)feasibility of otherwise-similar
attacks can vary. No big deal, just something to improve where we can.
For others reading just the list postings and for archival, this newer
Linux kernel commit is:
> author Borislav Petkov (AMD) <bp@...en8.de> 2023-08-11 23:38:24 +0200
> committer Borislav Petkov (AMD) <bp@...en8.de> 2023-08-14 11:02:50 +0200
>
> x86/CPU/AMD: Fix the DIV(0) initial fix attempt
>
> Initially, it was thought that doing an innocuous division in the #DE
> handler would take care to prevent any leaking of old data from the
> divider but by the time the fault is raised, the speculation has already
> advanced too far and such data could already have been used by younger
> operations.
>
> Therefore, do the innocuous division on every exit to userspace so that
> userspace doesn't see any potentially old data from integer divisions in
> kernel space.
>
> Do the same before VMRUN too, to protect host data from leaking into the
> guest too.
>
> Fixes: 77245f1c3c64 ("x86/CPU/AMD: Do not leak quotient data after a division by 0")
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> Cc: <stable@...nel.org>
> Link: https://lore.kernel.org/r/20230811213824.10025-1-bp@alien8.de
Alexander
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