Description

The hypervisor contains code to accelerate VGA memory accesses for HVM guests, when the (virtual) VGA is in "standard" mode. Locking involved there has an unusual discipline, leaving a lock acquired past the return from the function that acquired it. This behavior results in a problem when emulating an instruction with two memory accesses, both of which touch VGA memory (plus some further constraints which aren't relevant here). When emulating the 2nd access, the lock that is already being held would be attempted to be re-acquired, resulting in a deadlock. This deadlock was already found when the code was first introduced, but was analysed incorrectly and the fix was incomplete. Analysis in light of the new finding cannot find a way to make the existing locking discipline work. In staging, this logic has all been removed because it was discovered to be accidentally disabled since Xen 4.7. Therefore, we are fixing the locking problem by backporting the removal of most of the feature. Note that even with the feature disabled, the lock would still be acquired for any accesses to the VGA MMIO region.

Metrics

 

CVSS Version 4.0 CVSS Version 3.x CVSS Version 2.0

NVD enrichment efforts reference publicly available information to associate vector strings. CVSS information contributed by other sources is also displayed.

CVSS 4.0 Severity and Vector Strings:

NIST: NVD

N/A

NVD assessment not yet provided.

CVSS 3.x Severity and Vector Strings:

NIST: NVD

Base Score:  N/A

NVD assessment not yet provided.

ADP:  CISA-ADP

Base Score:  6.5 MEDIUM

Vector:  CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CVSS 2.0 Severity and Vector Strings:

NIST: NVD

Base Score: N/A

NVD assessment not yet provided.

AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CWE-667

http://www.openwall.com/lists/oss-security/2024/11/12/2

http://xenbits.xen.org/xsa/advisory-463.html

The hypervisor contains code to accelerate VGA memory accesses for HVM
guests, when the (virtual) VGA is in "standard" mode.  Locking involved
there has an unusual discipline, leaving a lock acquired past the
return from the function that acquired it.  This behavior results in a
problem when emulating an instruction with two memory accesses, both of
which touch VGA memory (plus some further constraints which aren't
relevant here).  When emulating the 2nd access, the lock that is already
being he

https://xenbits.xenproject.org/xsa/advisory-463.html