DescriptionMishandling of guest SSBD selection on AMD hardware The current logic to set SSBD on AMD Family 17h and Hygon Family 18h processors requires that the setting of SSBD is coordinated at a core level, as the setting is shared between threads. Logic was introduced to keep track of how many threads require SSBD active in order to coordinate it, such logic relies on using a per-core counter of threads that have SSBD active. When running on the mentioned hardware, it's possible for a guest to under or overflow the thread counter, because each write to VIRT_SPEC_CTRL.SSBD by the guest gets propagated to the helper that does the per-core active accounting. Underflowing the counter causes the value to get saturated, and thus attempts for guests running on the same core to set SSBD won't have effect because the hypervisor assumes it's already active.
SourceCVE (at NVD; CERT, LWN, oss-sec, fulldisc, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)
Debian Bugs1036298

Vulnerable and fixed packages

The table below lists information on source packages.

Source PackageReleaseVersionStatus
xen (PTS)jessie, jessie (lts)4.4.4lts5-0+deb8u1vulnerable
stretch (security), stretch (lts),
buster, buster (security)4.11.4+107-gef32c7afa2-1fixed
bullseye (security)4.14.5+94-ge49571868d-1fixed

The information below is based on the following data on fixed versions.

PackageTypeReleaseFixed VersionUrgencyOriginDebian Bugs
xensourcebuster(not affected)
xensourcebullseye(not affected)


[bullseye] - xen <not-affected> (Vulnerable code not present)
[buster] - xen <not-affected> (Vulnerable code not present)

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