UPDATES IN VERSION 2
The typical behaviour of singlestepping exceptions is determined at the
start of the instruction, with a #DB trap being raised at the end of the
SYSCALL (and SYSRET, although we don't implement it) behave differently
because the typical behaviour allows userspace to escalate its
privilege. (This difference in behaviour seems to be undocumented.)
Xen wrongly raised the exception based on the flags at the start of
Guest userspace which can invoke the instruction emulator can use this
flaw to escalate its privilege to that of the guest kernel.
All Xen versions are affected.
The vulnerability is only exposed to 64-bit x86 HVM guests.
On Xen 4.6 and earlier the vulnerability is exposed to all guest user
processes, including unprivileged processes, in such guests.
On Xen 4.7 and later, the vulnerability is exposed only to guest user
processes granted a degree of privilege (such as direct hardware access)
by the guest administrator; or, to all user processes when the VM has
been explicitly configured with a non-default cpu vendor string (in
xm/xl, this would be done with a `cpuid=' domain config option).
A 64-bit guest kernel which uses an IST for #DB handling will most likely
mitigate the issue, but will have a single unexpected #DB exception
frame to deal with. This in practice means that Linux is not
The vulnerability is not exposed to 32-bit HVM guests. This is because
the emulation bug also matches real hardware behaviour, and a 32-bit
guest kernel using SYSCALL will already have to be using a Task Gate for
handling #DB to avoid being susceptible to an escalation of privilege.
The vulnerability is not exposed to PV guests.
ARM systems are not vulnerable.
There is no known mitigation.
Applying the appropriate attached patch resolves this issue.
xsa204-4.8.patch Xen 4.8.x
xsa204-4.7.patch Xen 4.7.x, Xen 4.6.x
xsa204-4.5.patch Xen 4.5.x, Xen 4.4.x
already fixed in tree, see bug 601986
Added to GLSA 201612-56