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Xen Security Advisory CVE-2017-14318 / XSA-232
Missing check for grant table
UPDATES IN VERSION 4
Added metadata file
The function `__gnttab_cache_flush` handles GNTTABOP_cache_flush grant
table operations. It checks to see if the calling domain is the owner
of the page that is to be operated on. If it is not, the owner's grant
table is checked to see if a grant mapping to the calling domain
exists for the page in question.
However, the function does not check to see if the owning domain
actually has a grant table or not. Some special domains, such as
`DOMID_XEN`, `DOMID_IO` and `DOMID_COW` are created without grant
tables. Hence, if __gnttab_cache_flush operates on a page owned by
these special domains, it will attempt to dereference a null pointer
in the domain struct.
The guest can get Xen to dereference a NULL pointer.
For ARM guests and x86 PV guests on systems with SMAP enabled, this will
cause a host crash (denial-of-service).
For x86 PV guests on systems without SMAP enabled, an attacker can map
a crafted grant structure at virtual address 0. This can be leveraged
to increment an arbitrary virtual address, which can then probably be
leveraged into a full privilege escalation.
All versions of Xen since Xen 4.5 are vulnerable.
x86 HVM guests do not expose the vulnerability.
ARM guests and x86 PV guests on systems with SMAP enabled are only
vulnerable to a Denial-of-Service (host crash).
x86 PV guests on systems without SMAP running are vulnerable to a
Hardware supporting Supervisor Mode Access Prevention (Intel Broadwell,
AMD Zen) can mitigate the privilege escalation to a DoS.
This issue was discovered by Matthew Daley.
Applying the attached patch resolves this issue.
xsa232.patch xen-unstable, 4.9, 4.8, 4.7, 4.6, 4.5
$ sha256sum xsa232*
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is then no longer applicable.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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