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                             CERT-Renater

                 Note d'Information No. 2018/VULN248
_____________________________________________________________________

DATE                : 26/07/2018

HARDWARE PLATFORM(S): 64-bit x86.

OPERATING SYSTEM(S): 64-bit x86 PV Linux systems.

=====================================================================
https://xenbits.xen.org/xsa/advisory-274.html
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                    Xen Security Advisory XSA-274

         Linux: Uninitialized state in PV syscall return path

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

Linux has a `failsafe` callback, invoked by Xen under certain
conditions.  Normally in this failsafe callback, error_entry is paired
with error_exit; and error_entry uses %ebx to communicate to
error_exit whether to use the user or kernel return path.

Unfortunately, on 64-bit PV Xen on x86, error_exit is called without
error_entry being called first, leaving %ebx with an invalid value.

IMPACT
======

A rogue user-space program could crash a guest kernel.  Privilege
escalation cannot be ruled out.

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

Only 64-bit x86 PV Linux systems are vulnerable.

All versions of Linux are vulnerable.

MITIGATION
==========

Switching to HVM or PVH guests will mitigate this issue.

CREDITS
=======

This issue was discovered by M. Vefa Bicakci, and recognized as a
security issue by Andy Lutorminski.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

NB this patch has not been accepted into Linux upstream yet.  An
updated advisory will be sent if the fix upstreamed looks
significantly different.

xsa274-linux-4.17.patch           Linux 4.17

$ sha256sum xsa274*
0c30cb13d1d573f446c8cb8d4824ffad8ef9149a7589a19ef9bcc83c07bddcf5
xsa274-linux-4.17.patch
$

NOTE ON THE LACK OF EMBARGO
===========================

The patch for this issue was published on linux-kernel without being
first reported to the XenProject Security Team.


=========================================================
+ CERT-RENATER        | tel : 01-53-94-20-44            +
+ 23/25 Rue Daviel    | fax : 01-53-94-20-41            +
+ 75013 Paris         | email:cert@support.renater.fr   +
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