Xen Security Advisory 366 v1 - missed flush in XSA-321 backport

Xen Security Advisory 366 v1 - missed flush in XSA-321 backport

[Xen.org security team]

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                    Xen Security Advisory XSA-366

                   missed flush in XSA-321 backport

ISSUE DESCRIPTION
=================

An oversight was made when backporting XSA-320, leading entries in the
IOMMU not being properly updated under certain circumstances.

IMPACT
======

A malicious guest may be able to retain read/write DMA access to
frames returned to Xen's free pool, and later reused for another
purpose.  Host crashes (leading to a Denial of Service) and privilege
escalation cannot be ruled out.

VULNERABLE SYSTEMS
==================

Xen versions up to 4.11, from at least 3.2 onwards, are affected.  Xen
versions 4.12 and newer are not affected.

Only x86 Intel systems are affected.  x86 AMD as well as Arm systems are
not affected.

Only x86 HVM guests using hardware assisted paging (HAP), having a
passed through PCI device assigned, and having page table sharing
enabled can leverage the vulnerability.  Note that page table
sharing will be enabled (by default) only if Xen considers IOMMU and
CPU large page size support compatible.

MITIGATION
==========

Suppressing the use of page table sharing will avoid the vulnerability
(command line option "iommu=no-sharept").

Suppressing the use of large HAP pages will avoid the vulnerability
(command line options "hap_2mb=no hap_1gb=no").

Not passing through PCI devices to HVM guests will avoid the
vulnerability.

CREDITS
=======

This issue was reported as a bug by M. Vefa Bicakci, and recognized as
a security issue by Roger Pau Monne of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa366-4.11.patch      Xen 4.11.x

$ sha256sum xsa366*
3131c9487b9446655e2e21df4ccf1e003bec471881396d7b2b1a0939f5cbae96  xsa366.meta
8c8c18ca8425e6167535c3cf774ffeb9dcb4572e81c8d2ff4a73fefede2d4d94  xsa366-4.11.patch
$

NOTE REGARDING LACK OF EMBARGO
==============================

This was reported and debugged publicly, before the security
implications were apparent.
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[-- Attachment #2: xsa366.meta --]
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{
  "XSA": 366,
  "SupportedVersions": [
    "4.11"
  ],
  "Trees": [
    "xen"
  ],
  "Recipes": {
    "4.11": {
      "Recipes": {
        "xen": {
          "StableRef": "80cad584fb4c2599ae174226e2c913bb23df3bfa",
          "Prereqs": [],
          "Patches": [
            "xsa366-4.11.patch"
          ]
        }
      }
    }
  }
}

[-- Attachment #3: xsa366-4.11.patch --]
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From: Roger Pau Monne <roger.pau@citrix.com>
Subject: x86/ept: fix missing IOMMU flush in atomic_write_ept_entry

Backport of XSA-321 missed a flush in atomic_write_ept_entry when
level was different than 0. Such omission will undermine the fix for
XSA-321, because page table entries cached in the IOMMU can get out
of sync and contain stale entries.

Fix this by slightly re-arranging the code to prevent the early return
when level is different that 0. Note that the early return is just an
optimization because foreign entries cannot have level > 0.

This is XSA-366.

Reported-by: M. Vefa Bicakci <m.v.b@runbox.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
 xen/arch/x86/mm/p2m-ept.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
index 036771f43c..fde2f5f7e3 100644
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -53,12 +53,7 @@ static int atomic_write_ept_entry(ept_entry_t *entryptr, ept_entry_t new,
     bool_t check_foreign = (new.mfn != entryptr->mfn ||
                             new.sa_p2mt != entryptr->sa_p2mt);
 
-    if ( level )
-    {
-        ASSERT(!is_epte_superpage(&new) || !p2m_is_foreign(new.sa_p2mt));
-        write_atomic(&entryptr->epte, new.epte);
-        return 0;
-    }
+    ASSERT(!level || !is_epte_superpage(&new) || !p2m_is_foreign(new.sa_p2mt));
 
     if ( unlikely(p2m_is_foreign(new.sa_p2mt)) )
     {