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:
Doc Type: If docs needed, set a value
Doc Text:
A logic flaw was found in the handling of event channel operations in Xen. Operations such as resetting all event channels may involve decreasing one of the bounds checked when determining validity. This flaw allows an unprivileged guest to crash Xen, leading to a denial of service (DoS) for the entire system. The highest threat from this vulnerability is to system availability.
Clone Of:
Environment:
Last Closed: 2020-09-22 20:41:26 UTC


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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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