'007' code helps stop Spectre exploits before they exist

Singaporeans boffins offer Spectre-protector as Fortinet ponders Android inoculation

Black hats haven't yet found a way to mass-exploit the Spectre vulnerability – but mitigations are already arriving.

Beyond chip vendor and operating system patches, there remain reasons to seek out additional defences: there are still circumstances in which protective coverage is incomplete – and over in the world of Android phones, updates dribble out slowly.

Be of good heart, sysadmins. At arXiv, Singaporean and US researchers have published work, appropriately dubbed "007", which checks code to see if it's trying to exploit Spectre; and at Virus Bulletin, Fortinet's Axelle Apvrille takes a look at the bug from an Android point of view.

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Apvrille's work backs up what we've heard from other researchers: so far, Spectre exploitation is theoretical, with no exploits in the wild. She wrote that while there was a flurry of “Spectre exploit” stories based on AV-Test sample collection, it turned out that all of the reported samples were proofs-of-concept rather than genuine malware.

She adds: “there is a significant difference between a PoC of Spectre and a piece of malware using Spectre. Turning a PoC into a malicious executable is far from a trivial process.”

That doesn't make this kind of work pointless, though, since it's a good thing to stay ahead of whatever nasties black hats might devise.

In developing a detection technique, Apvrille's second conclusion was also good news: an attack against Spectre, she found, seems relatively easy to detect.

She wrote that “we had expected several false positives with this signature, but that was not the case: this imperfect signature turns out to be quite good in practice.”

The signature Apvrille searched for (using the in-practice impracticably-slow technique of searching whole binaries) was to identify “Flush+Reload cache attacks in ELF x86-64 executables”.

Although slow, that technique detected all of the viable samples in the proof-of-concept code gathered by AV-Test. Those that weren't successfully scanned, it turned out, wouldn't have worked anyway: “they were all damaged: the cache flush instruction was missing”.

And there's yet more good news for Android users: all of the proof-of-concept samples so far identified are for x86-64 architectures, and code doesn't easily port from there to ARMv7 architectures.

Double-oh Seven

The paper that landed at arXiv also seeks to detect code that attacks Spectre, at a generic level the authors describe as a “binary analysis framework to check and fix code snippets against potential vulnerability to Spectre attacks”.

Such things already exist, but the authors – Guanhua Wang, Tulika Mitra, and Abhik Roychoudhury from the National University of Singapore; Sudipta Chattopadhyay from the Singapore University of Technology and Design; and Ivan Gotovchits of Carnegie-Mellon University – explain that they impose heavy overheads, while their 007 framework imposed less than two per cent overhead, as measured by GNU Core Utilities.

They also claim to have detected “fourteen out of the fifteen Spectre vulnerable code patterns proposed by Paul Kocher, a feat that could not be achieved by the Spectre mitigation in C/C++ compiler proposed by Microsoft” (Kocher was one of the discoverers of Spectre, and he wrote this critique of Microsoft's C/C++ compiler fixes).

In the abstract, the 007 crew says their approach includes: “control flow extraction, taint analysis and address analysis to detect tainted conditional branches and their ability to impact memory accesses. Fixing is achieved by selectively inserting a small number of fences, instead of inserting fences after every conditional branch”.

The detection algorithm proposed in 007 is shown below (from the paper).

Input: P: Program binary
Output: Φ: A set of triplets of the form ⟨CB, IM1, IM2⟩ capturing Spectre vulnerabilities
1: Φ ← ∅;
2: TS.policy ← VtoV ▷ Taint policy set value-to-value
3: step ← None ▷ Initialize Spectre detection stage
4: Let inst be the first instruction of P
5: while inst , ex it do
6:              GS ← Interpreter.exe(inst) ▷ GS: Global State
7:              TaintEngine.taint(inst, GS) ▷ propagate taints
8:              if τ (inst) then ▷ oo7 is invoked only for tainted instruction
9:                      DS ← oo7.check(inst) ▷ DS: Detector State
10:     end if
11:     inst ← P.next() ▷ fetch next instruction
12: end while
13: procedure oo7.check(inst)
14:     step ← DS.step() ▷ Checks the stage of detection
15:     if br(inst) then ▷ check for CB
16:             DS ← DS.setCB(inst) ▷ recognize that inst might capture CB
17:             step ← STEP_CB ▷ progress the detection stage to CB
18:             TS.policy ← PtoV ▷ enable pointer-to-value taint
19:     end if
20:     if (load(inst) ∧ step = STEP_CB) then
21:             cb ← DS.CB() ▷ get CB from detection state
22:             if (Dep(cb, inst) ∧ ∆(cb, inst) ≤ SEW) then ▷ check for IM1
23:                     DS ← DS.setIM1(inst) ▷ recognize that inst might capture IM1
24:                     step ← STEP_IM1 ▷ progress the detection stage IM1
25:             end if
26:     end if
27:     if (mem(inst) ∧ step = STEP_IM1) then
28:             DS ← DS.setCB(inst) ▷ get CB from detection state
29:             if (Dep(cb, inst) ∧ ∆(cb, inst) ≤ SEW) then ▷ check for IM2
30:                     DS ← DS.setIM2(inst) ▷ recognize that inst might capture IM2
31:                     Φ ∪ = ⟨DS.CB(), DS.IM1(), DS.IM2()⟩ ▷ catch Spectre
32:                     step ← None ▷ reset checker
33:                     TS.policy ← VtoV ▷ disable pointer-to-value taint
34:                     end if
35:     end if
36:     if (step = STEP_CB ∧ ∆ (DS.CB(), inst) > SEW) then ▷ Outside SEW
37:             step ← None ▷ Reset detection beyond speculation window
38:             TS.policy ← VtoV
39:     end if
40:     if (step = STEP_IM1 ∧ ∆ (DS.CB(), inst) > SEW) then ▷ Outside SEW
41:             step ← None ▷ Reset detection beyond speculation window
42:             TS.policy ← VtoV
43:     end if
44:     return DS
45: end procedure

The researchers note that their detection code is available on request from the National University of Singapore's Website. ®




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