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Xen Security Advisory CVE-2017-5753,CVE-2017-5715,CVE-2017-5754 / XSA-254
Information leak via side effects of speculative execution
UPDATES IN VERSION 8
PVH shim ("Comet") is now available for Xen 4.8.
Fixes for two bugs in PVH shim "Comet": one relating to shim
initialisation, which can cause hangs during guest boot shortly after
host boot(!), and one to make qemu PV backends work in PVH mode.
Thanks to the respective contributors.
We are longer inclined to port the "Comet" patches to Xen 4.9. If
this causes you a problem please let us know by contacting us:
To: [hidden email]; CC: [hidden email]
Processors give the illusion of a sequence of instructions executed
one-by-one. However, in order to most efficiently use cpu resources,
modern superscalar processors actually begin executing many
instructions in parallel. In cases where instructions depend on the
result of previous instructions or checks which have not yet
completed, execution happens based on guesses about what the outcome
will be. If the guess is correct, execution has been sped up. If the
guess is incorrect, partially-executed instructions are cancelled and
architectural state changes (to registers, memory, and so on)
reverted; but the whole process is no slower than if no guess had been
made at all. This is sometimes called "speculative execution".
Unfortunately, although architectural state is rolled back, there are
other side effects, such as changes to TLB or cache state, which are
not rolled back. These side effects can subsequently be detected by
an attacker to determine information about what happened during the
speculative execution phase. If an attacker can cause speculative
execution to access sensitive memory areas, they may be able to infer
what that sensitive memory contained.
Furthermore, these guesses can often be 'poisoned', such that attacker
can cause logic to reliably 'guess' the way the attacker chooses.
This advisory discusses three ways to cause speculative execution to
access sensitive memory areas (named here according to the
discoverer's naming scheme):
"Bounds-check bypass" (aka SP1, "Variant 1", Spectre CVE-2017-5753):
Poison the branch predictor, such that victim code is speculatively
executed past boundary and security checks. This would allow an
attacker to, for instance, cause speculative code in the normal
hypercall / emulation path to execute with wild array indexes.
"Branch Target Injection" (aka SP2, "Variant 2", Spectre CVE-2017-5715):
Poison the branch predictor. Well-abstracted code often involves
calling function pointers via indirect branches; reading these
function pointers may involve a (slow) memory access, so the CPU
attempts to guess where indirect branches will lead. Poisoning this
enables an attacker to speculatively branch to any code that is
executable by the victim (eg, anywhere in the hypervisor).
"Rogue Data Load" (aka SP3, "Variant 3", Meltdown, CVE-2017-5754):
On some processors, certain pagetable permission checks only happen
when the instruction is retired; effectively meaning that speculative
execution is not subject to pagetable permission checks. On such
processors, an attacker can speculatively execute arbitrary code in
userspace with, effectively, the highest privilege level.
More information is available here:
Additional Xen-specific background:
Xen hypervisors on most systems map all of physical RAM, so code
speculatively executed in a hypervisor context can read all of system
When running PV guests, the guest and the hypervisor share the address
space; guest kernels run in a lower privilege level, and Xen runs in
the highest privilege level. (x86 HVM and PVH guests, and ARM guests,
run in a separate address space to the hypervisor.) However, only
64-bit PV guests can generate addresses large enough to point to
Xen guests may be able to infer the contents of arbitrary host memory,
including memory assigned to other guests.
An attacker's choice of code to speculatively execute (and thus the
ease of extracting useful information) goes up with the numbers. For
SP1, an attacker is limited to windows of code after bound checks of
user-supplied indexes. For SP2, the attacker will in many cases will
be limited to executing arbitrary pre-existing code inside of Xen.
For SP3 (and other cases for SP2), an attacker can write arbitrary
code to speculatively execute.
Additionally, in general, attacks within a guest (from guest user to
guest kernel) will be the same as on real hardware. Consult your
operating system provider for more information.
NOTE ON TIMING
This vulnerability was originally scheduled to be made public on 9
January. It was accelerated at the request of the discloser due to
one of the issues being made public.
Systems running all versions of Xen are affected.
For SP1 and SP2, both Intel and AMD are vulnerable. Vulnerability of
ARM processors to SP1 and SP2 varies by model and manufacturer. ARM
has information on affected models on the following website:
For SP3, only Intel processors are vulnerable. (The hypervisor cannot
be attacked using SP3 on any ARM processors, even those that are
listed as affected by SP3.)
Furthermore, only 64-bit PV guests can exploit SP3 against Xen. PVH,
HVM, and 32-bit PV guests cannot exploit SP3.
There is no mitigation for SP1 and SP2.
SP3 can be mitigated by running guests in HVM or PVH mode.
(Within-guest attacks are still possible unless the guest OS has also
been updated with an SP3 mitigation series such as KPTI/Kaiser.)
For guests with legacy PV kernels which cannot be run in HVM or PVH
mode directly, we have developed two "shim" hypervisors that allow PV
guests to run in HVM mode or PVH mode. This prevents attacks on the
host, but it leaves the guest vulnerable to Meltdown attacks by its
own unprivileged processes, even if the guest OS has KPTI or similar
The HVM shim (codenamed "Vixen") is available now, as is the PVH shim
(codenamed "Comet") for Xen 4.10 and Xen 4.8. Please read
README.which-shim to determine which shim is suitable for you.
$ sha256sum xsa254*/*
There is no available resolution for SP1. A solution may be available
in the future.
We are working on patches which mitigate SP2 but these are not
currently available. Given that the vulnerabilities are now public,
these will be developed and published in public, initially via
NOTE ON LACK OF EMBARGO
The timetable and process were set by the discloser.
After the intensive initial response period for these vulnerabilities
is over, we will prepare and publish a full timeline, as we have done
in a handful of other cases of significant public interest where we
saw opportunities for process improvement.
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