x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests

ISSUE DESCRIPTION

32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen’s novel approach to virtualization. In AMD64, Xen had to use a different implementation approach, so Xen does not use ring 1 to support 64-bit guests. With the focus now being on 64-bit systems, and the availability of explicit hardware support for virtualization, fixing speculation issues in ring 1 is not a priority for processor companies.
Indirect Branch Restricted Speculation (IBRS) is an architectural x86 extension put together to combat speculative execution sidechannel attacks, including Spectre v2. It was retrofitted in microcode to existing CPUs.
For more details on Spectre v2, see: <a href=“http://xenbits.xen.org/xsa/advisory-254.html”>http://xenbits.xen.org/xsa/advisory-254.html</a>
However, IBRS does not architecturally protect ring 0 from predictions learnt in ring 1.
For more details, see: <a href=“https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation”>https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation</a>
Similar situations may exist with other mitigations for other kinds of speculative execution attacks. The situation is quite likely to be similar for speculative execution attacks which have yet to be discovered, disclosed, or mitigated.

IMPACT

A malicious 32-bit guest kernel may be able to mount a Spectre v2 attack against Xen, despite the presence hardware protections being active.
It therefore might be able to infer the contents of arbitrary host memory, including memory assigned to other guests.

VULNERABLE SYSTEMS

Systems running all versions of Xen are affected.
Only x86 systems are vulnerable, and only CPUs which are potentially vulnerable to Spectre v2. Consult your hardware manufacturer.
The vulnerability can only be exploited by 32-bit PV guests which are not run in PV-Shim.