Bug 1879553 (CVE-2020-25597) - CVE-2020-25597 xen: once valid event channels may not turn invalid (XSA-338)
Summary: CVE-2020-25597 xen: once valid event channels may not turn invalid (XSA-338)
Keywords:
Status: CLOSED NOTABUG
Alias: CVE-2020-25597
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
medium
medium
Target Milestone: ---
Assignee: Red Hat Product Security
QA Contact:
URL:
Whiteboard:
Depends On: 1881588
Blocks: 1877386
TreeView+ depends on / blocked
 
Reported: 2020-09-16 14:05 UTC by Guilherme de Almeida Suckevicz
Modified: 2021-02-16 19:15 UTC (History)
25 users (show)

Fixed In Version:
Clone Of:
Environment:
Last Closed: 2020-09-22 20:41:26 UTC
Embargoed:

Attachments (Terms of Use)

Description Guilherme de Almeida Suckevicz 2020-09-16 14:05:37 UTC 
Logic in the handling of event channel operations in Xen assumes that an event channel, once valid, will not become invalid over the life time of a guest. However, operations like the resetting of all event channels may involve decreasing one of the bounds checked when determining validity. This may lead to bug checks triggering, crashing the host.
Comment 1 Mauro Matteo Cascella 2020-09-16 16:13:05 UTC 
Acknowledgments:

Name: the Xen project
Comment 2 Mauro Matteo Cascella 2020-09-17 13:05:50 UTC 
Mitigation:

The issue can be avoided by reducing the number of event channels available to the guest to no more than 1023.  For example, setting `max_event_channels=1023` in the xl domain configuration, or deleting any existing setting (since 1023 is the default for xl/libxl).

For ARM systems, any limit no more than 4095 is safe. For 64-bit x86 PV guests, any limit no more than 4095 is likewise safe if the host configuration prevents the guest administrator from substituting and running a 32-bit kernel (and thereby putting the guest into 32-bit PV mode).
Comment 3 Mauro Matteo Cascella 2020-09-17 13:07:59 UTC 
Statement:

All Xen versions from 4.4 onwards are vulnerable. Red Hat Enterprise Linux 5 is not affected by this flaw, as it shipped an older version of Xen.
Comment 4 Mauro Matteo Cascella 2020-09-22 17:56:46 UTC 
Created xen tracking bugs for this issue:

Affects: fedora-all [bug 1881588]
Comment 5 Product Security DevOps Team 2020-09-22 20:41:26 UTC 
This bug is now closed. Further updates for individual products will be reflected on the CVE page(s):

https://access.redhat.com/security/cve/cve-2020-25597
Comment 6 Mauro Matteo Cascella 2020-09-23 07:36:08 UTC 
External References:

https://xenbits.xen.org/xsa/advisory-338.html
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