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    <p>It's not much but I found a way to locate simple integer variables and strings.</p> <ol> <li>Run js shell with gdb</li> <li>Place a breakpoint Following the instructions <a href="https://developer.mozilla.org/en-US/docs/SpiderMonkey/Hacking_Tips#Printing_the_generated_assembly_code_%28from_gdb%29" rel="nofollow">here</a> </li> <li>do a x/50i $pc-1 and start!</li> </ol> <h2>For integers:</h2> <p><strong>Javascript code</strong></p> <pre><code>var sum =10; for (var i = 0; i &lt; 100000 ; i++ ) { sum = sum + 1; } </code></pre> <p><strong>Generated code</strong></p> <pre><code> 0x7ffff7ff34a7: movabs $0x7ffff5e4c060,%rax 0x7ffff7ff34b1: mov 0x10(%rax),%rax 0x7ffff7ff34b5: movabs $0x1670b98,%r11 0x7ffff7ff34bf: cmpl $0x0,(%r11) 0x7ffff7ff34c3: jne 0x7ffff7ff3542 0x7ffff7ff34c9: mov 0x6c0(%rax),%ecx -- Load var i 0x7ffff7ff34cf: cmp $0x186a0,%ecx -- compare with 100000 0x7ffff7ff34d5: jge 0x7ffff7ff34fe -- jump greater or equal [loop end] 0x7ffff7ff34db: mov 0x6b8(%rax),%edx -- load var sum 0x7ffff7ff34e1: add $0x1,%edx -- +1 to sum 0x7ffff7ff34e4: jo 0x7ffff7ff3561 0x7ffff7ff34ea: mov %edx,0x6b8(%rax) -- store sum 0x7ffff7ff34f0: add $0x1,%ecx -- +1 to i 0x7ffff7ff34f3: mov %ecx,0x6c0(%rax) -- store i 0x7ffff7ff34f9: jmpq 0x7ffff7ff34b5 -- continue loop 0x7ffff7ff34fe: movabs $0xfff9000000000000,%rcx 0x7ffff7ff3508: retq </code></pre> <p><strong>Now we check the memory</strong></p> <p>var i -- 0x1696230 + 0x6c0 = 0x16968f0 var sum -- 0x1696230 + 0x6b8 = 0x16968e8</p> <pre><code>0x00007ffff7ff34b5 (gdb) info registers rax 0x1696230 (gdb) x/w 0x16968f0: 0x0000044a (gdb) x/w 0x16968e8: 0x00000454 </code></pre> <p>0x0000044a = hex( 44a ) = dec( 1098 ) 0x00000454 = hex( 454 ) = dec( 1108 )</p> <p>So, that's the way we find the memory address of an integer.</p> <h2>For strings:</h2> <p><strong>Javascript code</strong></p> <pre><code>for (var i = 0; i &lt; 100000; i++){ shell="ABCDEFG" } </code></pre> <p><em>info</em> ascii( "ABCDEFG" ) = hex( 41 42 43 44 45 46 47 )</p> <p><strong>Generated code</strong></p> <pre><code> 0x7ffff7ff341f: movabs $0x7ffff5e4c060,%rax 0x7ffff7ff3429: mov 0x10(%rax),%rax 0x7ffff7ff342d: movabs $0x1670b98,%r11 0x7ffff7ff3437: cmpl $0x0,(%r11) -- compare addr 0 0x7ffff7ff343b: jne 0x7ffff7ff34d7 -- jump not equal 0x7ffff7ff3441: mov 0x6b8(%rax),%ecx -- load var i 0x7ffff7ff3447: cmp $0x186a0,%ecx -- compare i with 100000 0x7ffff7ff344d: jge 0x7ffff7ff3493 -- jump greater or equal [loop end] 0x7ffff7ff3453: jmpq 0x7ffff7ff3470 -- continue loop 0x7ffff7ff3458: push %rdx 0x7ffff7ff3459: lea 0x6c0(%rax),%rdx 0x7ffff7ff3460: callq 0x7ffff7fe9a48 0x7ffff7ff3465: pop %rdx 0x7ffff7ff3466: jmpq 0x7ffff7ff3470 0x7ffff7ff346b: hlt 0x7ffff7ff3...: hlt -- filled with hlt 0x7ffff7ff346f: hlt 0x7ffff7ff3470: movabs $0xfffafffff5f3a280,%r11 -- Load address "shell" 0x7ffff7ff347a: mov %r11,0x6c0(%rax) -- store address to var shell 0x7ffff7ff3481: mov $0x1,%edx 0x7ffff7ff3486: add %ecx,%edx -- +1 to i 0x7ffff7ff3488: mov %edx,0x6b8(%rax) -- store var i 0x7ffff7ff348e: jmpq 0x7ffff7ff342d -- continue loop 0x7ffff7ff3493: movabs $0xfff9000000000000,%rcx 0x7ffff7ff349d: retq </code></pre> <p>var shell -- 0x1696230 + 0x6c0 = 0x16968f0</p> <pre><code>(gdb) info registers rax 0x1696230: 0xf5f3a280 (gdb) x/w 0x16968f0: 0xf5f3a280 (gdb) x/20w 0x7ffff5f3a280 0x7ffff5f3a280: 0x00000078 0x00000000 0xf5f3a290 0x00007fff 0x7ffff5f3a290: 0x00420041 0x00440043 0x00460045 0x00000047 0x7ffff5f3a2a0: 0x00000000 0x00000000 0x00000000 0x00000000 </code></pre> <p><strong>0x7ffff5f3a290: 0x00420041 0x00440043 0x00460045 0x00000047</strong> We got our string!</p>
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