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POImplementing a sandbox with custom stack on Windows 64-bit
 

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    <p>I am currently investigating how to implement a sandbox (similar to <a href="http://code.google.com/p/nativeclient">Google's NaCl project</a> ) where I can run untrusted x86 code (restricted instruction set) in such a way that it cannot harm the rest of my process.</p> <p>Unlike with NaCl, the untrusted code will not run in a separate process but the same process as the host application. So, one crucial step is to get Windows' structured exception handling right in order to catch errors (like invalid memory access or div by 0) and gracefully terminate the sandbox before Windows kills my host application. (NaCl doesn't face these issues. The sandbox is a separate process and simply gets killed in case of an error.)</p> <p>Furthermore, the sandboxed code should not use the host applications stack but run on some separate "stack" which is allocated by myself.</p> <p>Exactly this combination (exception handling in presence of a custom allocated stack) is twisting my mind. I have checked the language implementations of <a href="http://golang.org">Go</a> and <a href="http://factorcode.org/">Factor</a> which do similar things and with this help got something running.</p> <p>But there are still some open questions and uncertainties. So I thought I'll use the fantastic knowledge of Stack Overflow to get some opinions :-)</p> <p>The following is a working code snippet cut down to the core issues:</p> <p><strong>code.cpp</strong></p> <pre><code>#include &lt;Windows.h&gt; extern "C" void Sandbox(); // just a low level helper to print "msg" extern "C" void Write(const char* msg) { WriteFile(GetStdHandle(STD_OUTPUT_HANDLE), msg, (DWORD)strlen(msg), NULL, NULL); } // should be called first on error and continue exception handling LONG __stdcall GlobalExceptionHandler(_EXCEPTION_POINTERS*) { Write("GEH "); return EXCEPTION_CONTINUE_SEARCH; } // should be called afterwards on error and terminate the process // of course this is just a stub to simplify the issue // in real world it would just terminate the sandbox extern "C" EXCEPTION_DISPOSITION __stdcall FrameExceptionHandler( PEXCEPTION_RECORD, ULONG64, PCONTEXT, PVOID) { Write("FEH "); ExitProcess(42); } void main() { AddVectoredExceptionHandler(1, GlobalExceptionHandler); Sandbox(); // never reach this... ExitProcess(23); } </code></pre> <p><strong>code.asm</strong></p> <pre><code>EXTERN FrameExceptionHandler:PROC EXTERN malloc:PROC .code Handler: jmp FrameExceptionHandler Sandbox PROC FRAME : Handler ; function prologue compliant with Windows x86_64 calling conventions ; saves rsp to the "frame-pointer" r15 push r15 .PUSHREG r15 sub rsp, 20h .ALLOCSTACK(20h) mov r15, rsp .SETFRAME r15, 0h .ENDPROLOG ; set rsp to the top of a "heap allocated stack" of size 0x10000 bytes mov rcx, 10000h call malloc lea rsp, [rax+10000h] ; got this from implementation of the Go language runtime: ; while unwinding the stack, Windows sanity checks the values of ; RSP to be within stack-bounds. Of course RSP is set to our ; "heap allocated stack" and not within the bounds of what Windows ; thinks should be the stack. ; Fix this by adjusting StackBase and StackEnd in the TIB (thread ; information block), so that basically the stack is unbounded: ; StackBase = 0xffffffffffffffff, StackEnd = 0x0000000000000000 mov rcx, 0FFFFFFFFFFFFFFFFh mov gs:[008h], rcx mov rcx, 0 mov gs:[010h], rcx ; trigger an access error by reading invalid memory mov rax, 0DEADBEEFh mov rax, [rax] ; function epilogue - will never get here mov rax, 0 add rsp, 28h ret Sandbox ENDP end </code></pre> <p>Running this will print "GEH FEH " and then gracefully exit with code 42.</p> <p>Does anyone have more insight in this set <code>StackBase &amp; StackEnd</code> "hack"? I tried to narrow the stack limits to something like:</p> <pre><code> mov gs:[008h], rsp mov gs:[010h], rax ; rax is the address returned by malloc </code></pre> <p>But it does not work. It prints "GEH " and then crashes due to unhandled exceptions. <code>FrameExceptionHandler()</code> will never be executed.</p> <p>I also tried more relaxed boundaries that include the "heap allocated stack" as well as the stack allocated by Windows. But it doesn't help.</p> <p>Another question is, whether you know any other traps I can run into. For example I noticed that Windows does not like it if RSP is uneven (I guess because you can never get to an uneven RSP by doing 2/4/8 byte PUSHes and POPs on a 16-byte aligned stack pointer).</p> <p>Thanks, Jonas</p>
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    1. COHave you played around with [Sandboxie](http://www.sandboxie.com) at all? It might be helpful if you're just needing to have a sandbox utility handy.
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    2. CONo, unfortunately this does not help. I am not talking about running an untrusted application in my OS, but about running untrusted x86 code within my application. E.g. a plugin for new video codec in a video player. I don't want a buggy plugin to crash my application.
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    3. CO[Relying on exceptions may not be good enough - IsBadXxxPtr should really be called CrashProgramRandomly](http://blogs.msdn.com/b/oldnewthing/archive/2006/09/27/773741.aspx). Oddly enough, you can push and pop 16-bit values in 64-bit mode, while you can't do the same with 32-bit ones. If you don't want a buggy plugin to crash your process you may want to run the plugin in a separate/child process.
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      1. USAlexey Frunze
 

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