7.8 High
CVSS3
Attack Vector
LOCAL
Attack Complexity
HIGH
Privileges Required
LOW
User Interaction
NONE
Scope
CHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
4.4 Medium
CVSS2
Access Vector
LOCAL
Access Complexity
MEDIUM
Authentication
NONE
Confidentiality Impact
PARTIAL
Integrity Impact
PARTIAL
Availability Impact
PARTIAL
AV:L/AC:M/Au:N/C:P/I:P/A:P
0.0004 Low
EPSS
Percentile
13.4%
When mapping guest EPT (nested paging) tables, Xen would in some circumstances use a series of non-atomic bitfield writes.
Depending on the compiler version and optimisation flags, Xen might expose a dangerous partially-written PTE to the hardware, which an attacker might be able to race to exploit.
A guest administrator or perhaps even unprivileged guest user might be able to cause denial of service, data corruption, or privilege escalation.
Only systems using Intel CPUs are vulnerable. Sytems using AMD CPUs, and Arm systems, are not vulnerable.
Only systems using nested paging (“hap”, aka nested paging, aka in this case Intel EPT) are vulnerable.
Only HVM and PVH guests can exploit the vulnerability.
The presence and scope of the vulnerability depends on the precise optimisations performed by the compiler used to build Xen. If the compiler generates (a) a single 64-bit write, or (b) a series of read-modify-write operations which are in the same order as the source code, the hypervisor is not vulnerable.
For example, in one test build with gcc 8.3 with normal settings, the compiler generated multiple (unlocked) read-modify-write operations in source code order, which did not constitute a vulnerability.
We have not been able to survey compilers; consequently we cannot say which compiler(s) might produce vulnerable code (with which code generation options). The code clearly violates the C rules. So we have chosen to issue this advisory.
7.8 High
CVSS3
Attack Vector
LOCAL
Attack Complexity
HIGH
Privileges Required
LOW
User Interaction
NONE
Scope
CHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
4.4 Medium
CVSS2
Access Vector
LOCAL
Access Complexity
MEDIUM
Authentication
NONE
Confidentiality Impact
PARTIAL
Integrity Impact
PARTIAL
Availability Impact
PARTIAL
AV:L/AC:M/Au:N/C:P/I:P/A:P
0.0004 Low
EPSS
Percentile
13.4%