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 operation 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 [email protected] [email protected] @tklengyel
LibVMI http://libvmi.comDRAKVUF http://drakvuf.com
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