Virtual Machine Introspection Isolation Interpretation - - PowerPoint PPT Presentation

Virtual Machine Introspection

• Isolation
• Interpretation
• Interposition

Isolation

• From in-guest kernel/userspace
• Provided by Xen
• Buggy emulation blurres the line
• From trusted computing base (TCB)
• Possible via Xen Security Modules
• Move introspection system out from dom0!

Interposition

• Trap to Xen when something of interest

happens within the guest

• Enable optional hardware traps
• CLTS, HLT, LGDT, LIDT, LLDT, LTR, SGDT, MOV from

CR3, MOV from CR8, MOV to CR0, MOV to CR3, MOV to CR4, MOV to CR8, MOV DR, MWAIT, INT3, INT0, MTF, etc..

• See full list in Intel SDM 3c 25.1.3

Interposition

• Change access

permissions in EPT

• Trap violation into

Xen

• R/W/X
• With some caveats

EPT caveats

“An EPT violation that occurs during as a result of execution of a read-modify-write

• peration sets bit 1 (data write). Whether it

also sets bit 0 (data read) is implementation- specific and, for a given implementation, may differ for different kinds of read-modify- write operations.” - Intel SDM 3c

EPT caveats

• “Why can't the hardware report the true

characteristics right away?” - Jan Beulich

• “when spec says so, there is a reason but I

can't tell here. :-)” - Kevin Tian

• Well.. let's just mark all write volation as

read violation too..

• Patched in Xen 4.5

altp2m

• Add support for

multiple EPTs for second stage lookup!

• One table for

“restricted view”

• One table for “normal

view”

altp2m

• EPT pointer can be

swapped in the VMCS

• No need to change

EPT PTE permissions all the time

• No race condition

Interposition

• Once trapped to Xen, forward events
• Formerly known as mem_event
• Renamed and reworked as vm_event in 4.6
• Request/response via shared memory ring
• Monitor page used for VMI related events
• Two additional pages: memory sharing and

paging

vm_event & mem_access & monitor

• Let's keep track of subsystem names
• vm_event is the underlying request/response

mechanism

• mem_access memops control EPT
• monitor_op domctls control all other optional

VM execution traps

Event delivery structures in 4.6

• Defined in xen/vm_event.h public header
• Easily extendable and versioned
• No more hackery
• Event response can trigger specific behavior

without additional hypercalls

• Trigger emulation, singlestepping, swap altp2m...

VMI with Xen on ARM

• ARM has two-stage paging similar to EPT
• mem_access implemented for 4.6
• Some caveats:
• No singlestepping?
• Can be worked around but it's a pain
• Split-TLB ambiguities

ARM mem_access

• ARM PTEs have fewer software

programmable bits as compared to EPT

• ARM mem_access requires maintaining a

Radix-tree to keep track of PTEs with custom permissions

• Radix-tree keyed with GPA

ARM mem_access

• For a 2nd stage violation ARM provides the

faulting GVA

• GPA only provided if fault happened during

1st stage pagetable walk

• Xen needs to translate GVA to GPA to

perform Radix-tree lookup

ARM mem_access

• Native CPU instructions to perform GVA to

GPA translation

• Performs lookup as data-fetch access
• What if we trapped an instruction-fetch

access?

• In-guest translation hits iTLB
• Xen hits dTLB

• Split-TLB is a real rootkit problem
• ShadowWalker, MoRE, etc..
• Guest can load the iTLB with rootkit page

and dTLB with benign page

• Flushing the TLB does not help, iTLB

translation may be lost if PT no longer represents the cached translation

ARM Split-TLB problem

• Execution tracing with mem_access may be

problematic

• Use Secure Monitor Call (SMC) instruction

injection!

• Similar to 0xCC injection on x86
• TODO

ARM future work

• altp2m is primarily designed to be used with

Intel #VE

• VMCALL instruction to perform EPTP

switching from the guest

• Hybrid VMI

x86 future work

• Why aren't we using git pulls?
• Patches in mailinglist without branch-off point

specified

• Carving patches from mbox is a pain
• Start providing a public git branch for your

series!!

Lessons learnt

• Provide build-testing for the community
• It's a waste of time to wait for review on

something that's broken

• Check for style issues automatically?
• Travis-CI is OK but can time-out on large series
• https://github.com/tklengyel/xen/tree/travis

Lessons learnt

Thanks!

Tamas K Lengyel

tamas@tklengyel.com tlengyel@novetta.com @tklengyel LibVMI http://libvmi.com DRAKVUF http://drakvuf.com