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---===[ Qubes Security Bulletin #31 ]===---
June 20, 2017
Xen hypervisor vulnerabilities with unresearched impact (XSA 216-224)
Today the Xen Security Team has disclosed several Xen Security
Advisories (XSA 216-224). Impact ranges from leaks to system crashes
and potential privilege escalations. See also our commentary below.
Technical details
Xen Security Advisories 216 [1]:
| blkif responses leak backend stack data
| The block interface response structure has some discontiguous fields.
| Certain backends populate the structure fields of an otherwise
| uninitialized instance of this structure on their stacks, leaking
| data through the (internal or trailing) padding field.
| A malicious unprivileged guest may be able to obtain sensitive
| information from the host or other guests.
Xen Security Advisories 217 [2]:
| page transfer may allow PV guest to elevate privilege
| Domains controlling other domains are permitted to map pages owned by
| the domain being controlled. If the controlling domain unmaps such a
| page without flushing the TLB, and if soon after the domain being
| controlled transfers this page to another PV domain (via
| GNTTABOP_transfer or, indirectly, XENMEM_exchange), and that third
| domain uses the page as a page table, the controlling domain will have
| write access to a live page table until the applicable TLB entry is
| flushed or evicted. Note that the domain being controlled is
| necessarily HVM, while the controlling domain is PV.
| A malicious pair of guests may be able to access all of system memory,
| allowing for all of privilege escalation, host crashes, and
| information leaks.
Xen Security Advisories 218 [3]:
| Races in the grant table unmap code
| * When a grant had been mapped twice by a backend domain, and then
| unmapped by two concurrent unmap calls, the frontend may be informed
| that the page had no further mappings when the first call completed rather
| than when the second call completed.
| * A race triggerable by an unprivileged guest could cause a grant
| maptrack entry for grants to be "freed" twice. The ultimate effect of
| this would be for maptrack entries for a single domain to be re-used.
| For the first issue, for a short window of time, a malicious backend
| could still read and write memory that the frontend thought was its
| own again. Depending on the usage, this could be either an
| information leak, or a backend-to-frontend privilege escalation.
| The second issue is more difficult to analyze. It can probably cause
| reference counts to leak, preventing memory from being freed on domain
| destruction (denial-of-service), but information leakage or host
| privilege escalation cannot be ruled out.
Xen Security Advisories 219 [4]:
| x86: insufficient reference counts during shadow emulation
| When using shadow paging, writes to guest pagetables must be trapped and
| emulated, so the shadows can be suitably adjusted as well.
| When emulating the write, Xen maps the guests pagetable(s) to make the final
| adjustment and leave the guest's view of its state consistent.
| However, when mapping the frame, Xen drops the page reference before
| performing the write. This is a race window where the underlying frame can
| change ownership.
| One possible attack scenario is for the frame to change ownership and to be
| inserted into a PV guest's pagetables. At that point, the emulated write will
| be an unaudited modification to the PV pagetables whose value is under guest
| control.
| A malicious pair of guests may be able to elevate their privilege to that of
| Xen.
Xen Security Advisories 220 [5]:
| x86: PKRU and BND* leakage between vCPU-s
| There is an information leak, of control information mentioning
| pointers into guest address space; this may weaken address space
| randomisation and make other attacks easier.
| When an innocent guest acquires leaked state, it will run with
| incorrect protection state. This could weaken the protection intended
| by the MPX or PKU features, making other attacks easier which would
| otherwise be excluded; and the incorrect state could also cause a
| denial of service by preventing legitimate accesses.
Xen Security Advisories 221 [6]:
| NULL pointer deref in event channel poll
| When polling event channels, in general arbitrary port numbers can be
| specified. Specifically, there is no requirement that a polled event
| channel ports has ever been created. When the code was generalised
| from an earlier implementation, introducing some intermediate
| pointers, a check should have been made that these intermediate
| pointers are non-NULL. However, that check was omitted.
| A malicious or buggy guest may cause the hypervisor to access
| addresses it doesn't control, usually leading to a host crash (Denial
| of Service). Information leaks cannot be excluded.
Xen Security Advisories 222 [7]:
| stale P2M mappings due to insufficient error checking
| Certain actions require removing pages from a guest's P2M
| (Physical-to-Machine) mapping. When large pages are in use to map
| guest pages in the 2nd-stage page tables, such a removal operation may
| incur a memory allocation (to replace a large mapping with individual
| smaller ones). If this allocation fails, these errors are ignored by
| the callers, which would then continue and (for example) free the
| referenced page for reuse. This leaves the guest with a mapping to a
| page it shouldn't have access to.
| 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.
| A malicious guest may be able to access memory it doesn't own,
| potentially allowing privilege escalation, host crashes, or
| information leakage.
Xen Security Advisories 224 [8]:
| grant table operations mishandle reference counts
| * If a grant is mapped with both the GNTMAP_device_map and
| GNTMAP_host_map flags, but unmapped only with host_map, the device_map
| portion remains but the page reference counts are lowered as though it
| had been removed. This bug can be leveraged cause a page's reference
| counts and type counts to fall to zero while retaining writeable
| mappings to the page.
| * Under some specific conditions, if a grant is mapped with both the
| GNTMAP_device_map and GNTMAP_host_map flags, the operation may not
| grab sufficient type counts. When the grant is then unmapped, the
| type count will be erroneously reduced. This bug can be leveraged
| cause a page's reference counts and type counts to fall to zero while
| retaining writeable mappings to the page.
| * When a grant reference is given to an MMIO region (as opposed to a
| normal guest page), if the grant is mapped with only the
| GNTMAP_device_map flag set, a mapping is created at host_addr anyway.
| This does *not* cause reference counts to change, but there will be no
| record of this mapping, so it will not be considered when reporting
| whether the grant is still in use.
| For the worst issue, a PV guest could gain a writeable mapping of its
| own pagetable, allowing it to escalate its privileges to that of the
| host.
Commentary from the Qubes Security Team
The bugs discussed today seem difficult to exploit in practice.
Each require either some race condition to win (XSA 217, 218, 219),
control over more than one VM (XSA 218, 219), some memory allocation,
which is normally beyond attacker's control, to fail or happen in some
specific way (XSA 216, 217, 218, 219, 222, 224?), or a combination of
Additionally some bugs are believed to be limited to being leaks or
DoS only (XSA 216, 221), or affecting only intra-VM-security (XSA
Also, it's worth pointing out that 7 out of 8 of the bugs discussed
here (with XSA 222 being the exception) do not affect when running
only fully-virtualized PVH guests (which is where we have been going
to with Qubes 4.x, see [9]).
Compromise Recovery
Starting with Qubes 3.2 we offer Paranoid Backup Restore Mode, which
has been designed specifically to aid with recovery of a (potentially)
compromised Qubes OS system. Thus, if you believe your system might
have got compromised (perhaps because of the bugs discussed in this
bulletin), then you should read and follow the procedure described
The specific packages that resolve the problem discussed in this
bulletin are as follows:
For Qubes 3.2:
- Xen packages, version 4.6.5-28
- Kernel packages, version 4.4.67-13 (security-testing)
- Kernel packages, version 4.9.33-18 (current-testing)
The packages are to be installed in dom0 via the qubes-dom0-update
command or via the Qubes VM Manager.
A system restart will be required afterwards.
If you use Anti Evil Maid, you will need to reseal your secret
passphrase to new PCR values, as PCR18+19 will change due to the new
Xen and kernel binaries, and because of the regenerated initramfs.
These packages will migrate to the current (stable) repository over the
coming days after being tested by the community.
See original Xen Security Advisories.
The Qubes Security Team