Xen Security Advisory XSA-113
Guest effectable page reference leak in MMU_MACHPHYS_UPDATE handling
*** EMBARGOED UNTIL 2014-11-27 12:00 UTC ***
ISSUE DESCRIPTION
=================
An error handling path in the processing of MMU_MACHPHYS_UPDATE failed
to drop a page reference which was acquired in an earlier processing
step.
IMPACT
======
Malicious or buggy stub domain kernels or tool stacks otherwise living
outside of Domain0 can mount a denial of service attack which, if
successful, can affect the whole system.
Only domains controlling HVM guests can exploit this vulnerability.
(This includes domains providing hardware emulation services to HVM
guests.)
VULNERABLE SYSTEMS
==================
Xen versions from at least 3.2.x onwards are vulnerable on x86 systems.
Older versions have not been inspected. ARM systems are not vulnerable.
This vulnerability is only applicable to Xen systems using stub domains
or other forms of disaggregation of control domains for HVM guests.
MITIGATION
==========
Running only PV guests will avoid this issue.
(The security of a Xen system using stub domains is still better than
with a qemu-dm running as an unrestricted dom0 process. Therefore
users with these configurations should not switch to an unrestricted
dom0 qemu-dm.)
RESOLUTION
==========
Applying the attached patch resolves this issue.
xsa113.patch xen-unstable, Xen 4.4.x, Xen 4.3.x, Xen 4.2.x
$ sha256sum xsa113*.patch
a0f2b792a6b4648151f85fe13961b0bf309a568ed03e1b1d4ea01e4eabf1b18e xsa113.patch
$
x86/mm: fix a reference counting error in MMU_MACHPHYS_UPDATE
Any domain which can pass the XSM check against a translated guest can cause a
page reference to be leaked.
While shuffling the order of checks, drop the quite-pointless MEM_LOG(). This
brings the check in line with similar checks in the vicinity.
Discovered while reviewing the XSA-109/110 followup series.
This is XSA-113.
Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
Reviewed-by: Jan Beulich <jbeul...@suse.com>
Reviewed-by: Tim Deegan <t...@xen.org>
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3619,6 +3619,12 @@ long do_mmu_update(
case MMU_MACHPHYS_UPDATE:
+ if ( unlikely(paging_mode_translate(pg_owner)) )
+ {
+ rc = -EINVAL;
+ break;
+ }
+
mfn = req.ptr >> PAGE_SHIFT;
gpfn = req.val;
@@ -3638,13 +3644,6 @@ long do_mmu_update(
break;
}
- if ( unlikely(paging_mode_translate(pg_owner)) )
- {
- MEM_LOG("Mach-phys update on auto-translate guest");
- rc = -EINVAL;
- break;
- }
-
set_gpfn_from_mfn(mfn, gpfn);
paging_mark_dirty(pg_owner, mfn);
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