Date: Mon, 25 Sep 2023 18:10:05 +0100 From: Andrew Cooper <andrew.cooper3@...rix.com> To: Solar Designer <solar@...nwall.com>, oss-security@...ts.openwall.com Cc: "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 25/09/2023 5:36 pm, Solar Designer wrote: > Hi, > > Thank you Xen security team for indirectly bringing the various CPU > issues in here. This is very helpful, as your messages on them serve > two purposes at once - informing the community about issues fixed in Xen > (so directly on-topic here, with Xen being Open Source) and about the > CPU issues that typically also need to be mitigated by other projects. > > On Mon, Sep 25, 2023 at 04:05:37PM +0000, Xen. org security team wrote: >> Xen Security Advisory CVE-2023-20588 / XSA-439 >> >> x86/AMD: Divide speculative information leak >> >> ISSUE DESCRIPTION >> ================= >> >> In the Zen1 microarchitecure, there is one divider in the pipeline which >> services uops from both threads. In the case of #DE, the latched result >> from the previous DIV to execute will be forwarded speculatively. >> >> This is a covert channel that allows two threads to communicate without >> any system calls. In also allows userspace to obtain the result of the >> most recent DIV instruction executed (even speculatively) in the core, >> which can be from a higher privilege context. >> >> For more information, see: >> * https://www.amd.com/en/resources/product-security/bulletin/amd-sb-7008.html > The above link is wrong - it's for CVE-2023-20593 Zenbleed in Zen2. > > The correct link for CVE-2023-20588, the DIV bug in Zen1, appears to be: > > https://www.amd.com/en/resources/product-security/bulletin/amd-sb-7007.html Oops. I thought I'd fixed that, but apparently not. You're correct. I'll issue an update in a moment. > > 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. 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. Also the cross-thread nature is also poorly reported in public. Thanks, ~Andrew
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