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[Xen-devel] SWAPGSAttack and CVE-2019-1125
- To: "xen-devel@xxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 6 Aug 2019 18:00:04 +0100
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Hello,
With the embargo up, you'll surely all be pleased to see that this is
not a zeroday XSA.
https://www.bitdefender.com/SWAPGSAttack
There are two issues here.
1) Spectre attacks against SWAPGS in interrupt/exception handlers
This has been assigned CVE-2019-1125 and affects all 64bit operating
software using %gs base in the manner intended by the AMD64 spec.
Xen is not one of these systems, and is therefore not affected by the
issue. This is owing to a design decision made more than a decade ago.
In addition, 64bit PV guests running under Xen are not affected. SWAPGS
is a privileged instruction and the 64bit PV ABI was designed around
this restriction.
2) Stale segment leakage.
On some CPUs, the previous content of the segment registers is
recoverable under speculative execution after a context switch. This is
not secret information, so a CVE has not been assigned.
However, this is a very low-noise covert channel with some interesting
data transfer characteristics.
In the default case, a vCPU can observe the segment base values used by
the previous vCPU to execute on the current CPU. This may be a
different vCPU from the same guest, or from a different guest.
Furthermore, without Spectre v2 mitigations in place in Xen (and other
kernel level software), there are practical, albeit theoretical at this
point, ways to leak speculatively-obtained data via this covert channel,
rather than, e.g. via the L1D cache. I expect some research papers will
have fun with this avenue in the near future.
I have some patches to mitigate this covert channel which will be posted
to xen-devel when they are ready.
~Andrew
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