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Xen Security Advisory XSA-251
improper bug check in x86 log-dirty handling
UPDATES IN VERSION 2
Provide information for Xen 4.10-in-preparation branch in .meta.
Memory sharing, available to x86 HVM guests only, uses a special value
in the global machine to physical address translation table (M2P). PV
guests have full control over M2P entries corresponding to pages they
own. A bug check (specifically, an assertion that an M2P entry is not
the special "shared" indicator) was insufficiently qualified, and as a
consequence is triggerable by PV guests in log-dirty mode
(e.g. because of being live migrated).
A malicious or buggy PV guest may cause a hypervisor crash, resulting in
a Denial of Service (DoS) affecting the entire host.
Xen versions 4.0 and later are affected. Xen versions 3.4 and earlier
are not affected.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
x86 HVM guests cannot exploit this vulnerability.
Only x86 PV guests can exploit this vulnerability, and only when being
run in shadow mode. PV guests are typically run in shadow mode for live
migration, as well as for features like VM snapshot.
Note that save / restore does *not* use shadow mode, and so does not
expose this vulnerability. Some downstreams also include a "non-live
migration" feature, which also does not use shadow mode (and thus does
not expose this vulnerability).
Running only HVM guests avoids the vulnerability.
Avoiding live migration of x86 PV guests also avoids the vulnerability.
This issue was discovered by Jan Beulich of SUSE.
Applying the appropriate attached patch resolves this issue.
xsa251.patch xen-unstable, Xen 4.9.x
xsa251-4.8.patch Xen 4.8.x, Xen 4.7.x, Xen 4.6.x
xsa251-4.5.patch Xen 4.5.x
$ sha256sum xsa251*
DEPLOYMENT DURING EMBARGO
Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).
Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable. This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)
For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
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