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Date: Tue, 28 Nov 2017 12:00:24 +0000
From: security team <>
CC: security team <>
Subject: Xen Security Advisory 247 - Missing p2m error checking in PoD code

Hash: SHA256

                    Xen Security Advisory XSA-247
                              version 2

                 Missing p2m error checking in PoD code


Public release.


Certain actions require modification of entries in a guest's P2M
(Physical-to-Machine) table.  When large pages are in use for this
table, such an operation may incur a memory allocation (to replace a
large mapping with individual smaller ones).  If this allocation
fails, the p2m_set_entry() function will return an error.

Unfortunately, several places in the populate-on-demand code don't
check the return value of p2m_set_entry() to see if it succeeded.

In some cases, the operation was meant to remove an entry from the p2m
table.  If this removal fails, a malicious guest may engineer that the
page be returned to the Xen free list, making it available to be
allocated to another domain, while it retains a writable mapping to
the page.

In other cases, the operation was meant to remove special
populate-on-demand entries; if this removal fails, the internal
accounting becomes inconsistent and may eventually hit a BUG().

The allocation involved comes from a separate pool of memory created
when the domain is created; under normal operating conditions it never
fails, but a malicious guest may be able to engineer situations where
this pool is exhausted.


An unprivileged guest can retain a writable mapping of freed memory.
Depending on how this page is used, it could result in either an
information leak, or full privilege escalation.

Alternatively, an unprivileged guest can cause Xen to hit a BUG(),
causing a clean crash - ie, host-wide denial-of-service (DoS).


All systems from Xen 3.4 are vulnerable.

Only x86 systems are vulnerable.  ARM is not vulnerable.

x86 PV VMs cannot leverage the vulnerability.

Only systems with 2MiB or 1GiB HAP pages enabled are vulnerable.

The vulnerability is largely restricted to HVM guests which have been
constructed in Populate-on-Demand mode (i.e. with memory < maxmem):

x86 HVM domains without PoD (i.e. started with memory == maxmem, or
without mentioning "maxmem" in the guest config file) also cannot
leverage the vulnerability, in recent enough Xen versions:
  4.8.x and later: all versions safe if PoD not configured
  4.7.x: 4.7.1 and later safe if PoD not configured
  4.6.x: 4.6.4 and later safe if PoD not configured
  4.5.x: 4.5.4 and later safe if PoD not configured
  4.4.x and earlier: all versions vulnerable even if PoD not configured

The commit required to prevent this vulnerability when PoD
not configured is 2a99aa99fc84a45f505f84802af56b006d14c52e
  xen/physmap: Do not permit a guest to populate PoD pages for itself
and the corresponding backports.


Running only PV guests will avoid this issue.

Running HVM guests only in non-PoD mode (maxmem == memory) will also
avoid this issue.  NOTE: In older releases of Xen, an HVM guest can
create PoD entries itself; so this mitigation will not be effective.

Specifying "hap_1gb=0 hap_2mb=0" on the hypervisor command line will
also avoid the vulnerability.

Alternatively, running all x86 HVM guests in shadow mode will also
avoid this vulnerability.  (For example, by specifying "hap=0" in the
xl domain configuration file.)


This issue was discovered by George Dunlap of Citrix.


Applying the appropriate attached patch resolves this issue.

xsa247/*.patch           xen-unstable
xsa247-4.9/*.patch       Xen 4.9.x
xsa247-4.8/*.patch       Xen 4.8.x
xsa247-4.7/*.patch       Xen 4.7.x
xsa247-4.6/*.patch       Xen 4.6.x
xsa247-4.5/*.patch       Xen 4.5.x

$ sha256sum xsa247* xsa247*/*
e8fc454c35f429ab60b94c0e812f86fd2b3b37edfff2bfdcc13a7e13ebc2efbe  xsa247.meta
59e977d81ad85c25572b79db48d62b4f040026e88f51fe61051b7d30e97fad06  xsa247-4.5/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch
6221f5fc7899253888a1711e83436f1b8ddc51046ec920d83b7ea2f4266d13f7  xsa247-4.5/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch
f54c4984731f9138e522685e98359a0bb409146091fedb8b7beaac48b3460c22  xsa247-4.6/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch
258aaa76e164d70fbfead9de1370577c328dff78c09b81ac7b708fd5c530859a  xsa247-4.6/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch
85f0d5f3940bb27f84867b9ac227636a786519dfc1b35ad82f402f9c044ecac9  xsa247-4.7/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch
8f0d45b617e0b4c0c1ff490e84c6415f1444696d2afce09eeaa970fbedb8f4c3  xsa247-4.7/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch
580771a125aa577ff4c7607679ef5d8d6c668446f4573bf11e4fe6829d02d157  xsa247-4.8/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch
f88d252305d8229374f3fe25bae3c9ea165acab28be9908a1a9a816ae85170ac  xsa247-4.8/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch
5fe123448b8ea63f96495462a274d986016264acef4a81e555848ae0d38bd035  xsa247-4.9/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch
f6e061be4c6355a8d949d258bf1180ff607ce95ca40213cc0e2ee112db435ebd  xsa247-4.9/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch
4cfa8a89f0d2c4a7bf09f31df1e3050b3cce685efd01a17196e991e02d8dd61d  xsa247/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch
37d7f639af2c857d5232ab69aa9c576c37e6a46a7b246d0cd1cc6d05a93360ba  xsa247/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch


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
administrators, with ONE exception:

Removing the ability to boot in populate-on-demand mode is NOT
permitted during the embargo on public cloud systems.  This is because
doing so might alert attackers to the nature of the vulnerability.
Deployment of this mitigation is permitted only AFTER the embargo

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:
Version: GnuPG v1


Download attachment "xsa247.meta" of type "application/octet-stream" (1891 bytes)

Download attachment "xsa247-4.5/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch" of type "application/octet-stream" (6494 bytes)

Download attachment "xsa247-4.5/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch" of type "application/octet-stream" (4138 bytes)

Download attachment "xsa247-4.6/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch" of type "application/octet-stream" (6494 bytes)

Download attachment "xsa247-4.6/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch" of type "application/octet-stream" (4144 bytes)

Download attachment "xsa247-4.7/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch" of type "application/octet-stream" (6452 bytes)

Download attachment "xsa247-4.7/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch" of type "application/octet-stream" (4291 bytes)

Download attachment "xsa247-4.8/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch" of type "application/octet-stream" (6428 bytes)

Download attachment "xsa247-4.8/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch" of type "application/octet-stream" (4255 bytes)

Download attachment "xsa247-4.9/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch" of type "application/octet-stream" (6428 bytes)

Download attachment "xsa247-4.9/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch" of type "application/octet-stream" (4255 bytes)

Download attachment "xsa247/0001-p2m-Always-check-to-see-if-removing-a-p2m-entry-actu.patch" of type "application/octet-stream" (6211 bytes)

Download attachment "xsa247/0002-p2m-Check-return-value-of-p2m_set_entry-when-decreas.patch" of type "application/octet-stream" (4156 bytes)

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