pmtest5源码解析——特权级变换与堆栈切换实例详解

<span style="font-size:14px;">摘要：在<span lang="en-US">pmtest4</span>中，我们已经看到，对于非一致性代码，如何从低特权级转移向高特权级。但是我们该怎样从高特权级别，转移到低特权级别呢？本文，主要是为你解答这些疑问，展现实现这种特权级别转移的方法。另外，我们将在实践中探讨，为什么要设定<span lang="en-US">RPL</span>，是不是相对与<span lang="en-US">DPL</span>多此一举呢？在调试环节，我们将通过改动代码，排查相关错误来深入理解特权级检测机制。</span><h1><a target=_blank name="t0"></a><span style="font-size:18px;color:#ff0000;">一、预备理论</span></h1><div><span style="font-size:18px;color:#ff0000;"></span></div><span style="font-size:14px;"><strong><span lang="en-US">1.</span>关于堆栈</strong>我们都知道，可以通过<span lang="en-US">call</span>和<span lang="en-US">jmp</span>实现长跳转或者短跳转（段内转移或者段间转移）。但是<strong><span lang="en-US">jmp</span>和<span lang="en-US">call</span>毕竟是有分别的，其中<span lang="en-US">call</span>指令是会影响到堆栈的</strong>：对于短调用，<span lang="en-US">call</span>指令执行时，下一条指令的<span lang="en-US">eip</span>入栈，<span lang="en-US">ret</span>指令执行时候，这个<span lang="en-US">eip</span>就会从堆栈中弹出。短调用<span lang="en-US">call</span>指令执行前后堆栈的变化如下图所示（注意带有参数的<span lang="en-US">ret</span>指令）：长调用的情况与此相似，不同的是<span lang="en-US">call</span>指令执行时，被压栈的还有<span lang="en-US">cs</span>。<img src="http://img.blog.csdn.net/20140304172631843?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvdHJvY2hpbHVzZXM=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center" alt="" /></span>

<span style="font-size:14px;"><strong><span lang="en-US"></span></strong></span>

<span style="font-size:14px;"><strong><span lang="en-US">2.</span>特权级变换</strong>注意到：如果我们采用<span lang="en-US">call</span>指令进行长跳转，而且特权级别发生了变换，这个时候就发生了堆栈切换，比如从<span lang="en-US">A</span>切换到了<span lang="en-US">B</span>。但是我们的数据保存在<span lang="en-US">A</span>上面，<strong>如何从<span lang="en-US">B</span>中返回呢？原来，<span lang="en-US">intel</span>提供了一种机制，能够将<span lang="en-US">A</span>上的一些内容<span lang="en-US">copy</span>到<span lang="en-US">B</span>堆栈中。同时，<span lang="en-US">A</span>堆栈要切换到<span lang="en-US">B</span>堆栈，但是它怎么知道<span lang="en-US">B</span>堆栈在哪里呢？答案：原来这个内容保存在任务的<span lang="en-US">TSS</span>结构中，<span lang="en-US">TSS</span>是任务状态段。</strong>每个任务都最多有四个堆栈段，但是只有一个<span lang="en-US">esp</span>和<span lang="en-US">ss</span>，<span lang="en-US">TSS</span>就解决了堆栈切换方面的数据保存问题。<span lang="en-US">TSS</span>的相关介绍，可以参考：保护模式及其编程——任务切换<span lang="en-US"><a target=_blank target="_blank" href="http://blog.csdn.net/trochiluses/article/details/19768579">http://blog.csdn.net/trochiluses/article/details/19768579</a></span>。比如，我们目前是<span lang="en-US">ring 3</span>，需要转移到<span lang="en-US">ring 1</span>，那时，堆栈将自动切换到<span lang="en-US">ss1</span>和<span lang="en-US">esp1</span>。（由于只是从外层到内层切换，才从<span lang="en-US">TSS</span>中取得堆栈信息，所以，<span lang="en-US">TSS</span>中没有<span lang="en-US">ring 3</span>相关的堆栈信息。那么从内层向外层切换，如何获得目的地的信息呢）。切换过程中有关堆栈的处理，可以参考：保护模式及其编程——保护的详尽意义：通过调用门转移特权级<span lang="en-US"><a target=_blank target="_blank" href="http://blog.csdn.net/trochiluses/article/details/19573651">http://blog.csdn.net/trochiluses/article/details/19573651</a></span><strong>好了，我们已经知道<span lang="en-US">call------ret</span>分别实现特权级高低转化，可能我们平时一直先使用<span lang="en-US">call</span>，然后再使用<span lang="en-US">ret</span>，以至于你对它的印象好像<span lang="en-US">if</span>和<span lang="en-US">else</span>，实际上不是这样的，我们通过<span lang="en-US">retf</span>指令，单独使用就可以实现特权级别从<span lang="en-US">0</span>到<span lang="en-US">3</span>，从高到低。</strong></span><h1><a target=_blank name="t1"></a><span style="font-size:18px;color:#ff0000;">二、代码分析</span></h1><span style="font-size:14px;"><span lang="en-US">pmtest5.a</span>：我们需要增加一个<span lang="en-US">ring 3</span>的代码段，堆栈段，另外为了在<span lang="en-US">rign 3</span>下打印字符，需要将<span lang="en-US">video</span>段的<span lang="en-US">DPL</span>改成<span lang="en-US">3.</span>我们先来写一个测试版本<span lang="en-US">a:</span>程序执行流程和前面没有太大区别，在保护模式重打印字符串之后，我们让程序进入<span lang="en-US">ring 3</span>代码段，并且死循环，需要添加如下代码。</span>

<span style="font-size:14px;"></span><div class="dp-highlighter bg_plain"><div style="display: block;" class="bar"><div class="tools"><strong>[plain]</strong> <a target=_blank href="http://blog.csdn.net/trochiluses/article/details/20473941#" class="ViewSource" title="view plain">view plain</a><span data-mod="popu_168"> <a target=_blank href="http://blog.csdn.net/trochiluses/article/details/20473941#" class="CopyToClipboard" title="copy">copy</a></span><span data-mod="popu_169"> <a target=_blank href="http://blog.csdn.net/trochiluses/article/details/20473941#" class="PrintSource" title="print">print</a></span><a target=_blank href="http://blog.csdn.net/trochiluses/article/details/20473941#" class="About" title="?">?</a><span class="tracking-ad" data-mod="popu_167"><a target=_blank href="https://code.csdn.net/snippets/217080" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;" height="12" width="12" /></a></span><span class="tracking-ad" data-mod="popu_170"><a target=_blank href="https://code.csdn.net/snippets/217080/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;" height="12" width="12" /></a></span></div></div><ol start="1"><li class="alt"><span><span>push     SelectorStack3  </span></span></li><li><span>push     TopOfStack3  </span></li><li class="alt"><span>push     SelecorCodeRing3  </span></li><li><span>push    0  </span></li></ol></div>

<span style="font-size:14px;">        但是，如果我们去改动</span><span style="font-family:Arial,Helvetica,sans-serif;font-size:14px;">SelectorStack3和</span><span style="font-family:Arial,Helvetica,sans-serif;font-size:14px;">SelecorCodeRing3的描述符中RPL的字段，变成0，发现</span><span style="font-size:14px;">这个程序<span lang="en-US">pmtest5.asm</span>并没有像我们想象的那样，进入<span lang="en-US">ring3.</span>显然，<span lang="en-US">retf</span>的指令没有被正确执行。想一想，为什么？</span>

<span style="font-size:14px;"><span lang="en-US">      pmtest5.b</span>：在<span lang="en-US">a</span>的基础上，我们让程序在<span lang="en-US">ring3</span>代码段，进入死循环之前，调用调用门。在<span lang="en-US">pmtest4</span>的和<span lang="en-US">a</span>的基础上，我们需要做如下改动：改动调用门的描述符和选择符到<span lang="en-US">ring3</span>；添加<span lang="en-US">TSS</span>以便能够从低特权级别转移到高特权级，并进行相关初始化。      最后，我们让程序在调用门的目标段，也就是<span lang="en-US">ring 0</span>中，跳入局部任务。      总结一下，<span lang="en-US">pmtest5</span>的执行流程：实模式<span lang="en-US">---</span>保护模式<span lang="en-US">32b</span>代码段<span lang="en-US">ring 0---</span>通过<span lang="en-US">retf</span>进入<span lang="en-US">ring 3</span>的代码段——通过调用门，进入<span lang="en-US">ring 0</span>的目标代码段——通过<span lang="en-US">jmp</span>，跳转到<span lang="en-US">LDT</span>局部任务。</span><h1><a target=_blank name="t2"></a><span style="font-size:18px;color:#ff0000;">三、调试过程中发现的错误与分析</span></h1><span style="font-size:14px;">    <strong>1）为什么要有<span lang="en-US">RPL</span></strong>，我们都知道有“别名段”，那么别名段意味着什么呢？同一个段，可以对应着不同的<span lang="en-US">selector</span>，也就能够对应不同的<span lang="en-US">RPL</span>，这样就有利于实现自由的权限管理。      我们回忆，最开始的<span lang="en-US">retf</span>并没有生效，猜想到可能是权限的问题。我们从<span lang="en-US">ring0 </span>经过<span lang="en-US">retf </span>到<span lang="en-US">ring 3</span>；<span lang="en-US">call</span>，从<span lang="en-US">ring 3--->ring 0</span>，要求：<span lang="en-US">CPL<=DPL_G,RPL<=RPL_G,CPL>=DPL_B;</span>变化以后<span lang="en-US">CPL=DPL_B=0</span><span lang="en-US">retf</span>是相反的过程，<span lang="en-US">ring 0--->ring 3,</span>要求：<span lang="en-US">CPL<DPL_B</span>，<span lang="en-US">RPL<DPL_B!!!</span>注意，<span lang="en-US">retf</span>只能用于不同特权级之间，所以没有等于，这也就是为什么开始转移失败了。</span>

<span style="font-size:14px;"><strong>2）堆栈切换的特权级检查</strong></span>

<span style="font-size:14px;">    更改TSS段中，将ss对应部分改成selectorstack3，然后就能在调试bochs的时候收到如下信息：</span>

<span style="font-size:14px;">00082857633e[CPU0  ] call_protected: SS selector.rpl != CS descr.dpl    <strong>注明：开启出错信息的方法，在.bochsrc中添加对于log信息的注释<span lang="en-US">#log</span></strong></span><span style="font-family:Arial,Helvetica,sans-serif;"><span style="white-space:normal"></span></span>

<span style="font-size:14px;">程序代码如下：</span>

%include"head.inc"org 0100hjmp LABEL_BEGIN[SECTION .gdt];GDTbase,length,attrLABEL_GDT:      Descriptor         0,                0, 0 LABEL_DESC_NORMAL:Descriptor0,0ffffh,DA_DRWLABEL_DESC_CODE16:Descriptor0,0FFFFH,DA_CLABEL_DESC_DATA:Descriptor0,SegDataLen-1,DA_DRWLABEL_DESC_STACK:Descriptor0,TopOfStack,DA_DRWA+DA_32LABEL_DESC_CODE32:Descriptor0,SegCode32Len-1,DA_C+DA_32LABEL_DESC_CODE_DEST: Descriptor0,SegCodeDestLen-1,DA_C+DA_32LABEL_DESC_LDT:Descriptor0,LDTLen-1,DA_LDTLABEL_DESC_VIDEO:Descriptor0B8000h,0ffffh,DA_DRW + DA_DPL3LABEL_DESC_CODE_RING3:Descriptor0,SegCodeRing3Len-1,DA_C + DA_32 + DA_DPL3LABEL_DESC_STACK3:Descriptor  0,TopOfStack3,DA_DRWA + DA_32 + DA_DPL3LABEL_DESC_TSS:Descriptor0,TSSLen-1,DA_386TSSLABEL_CALL_GATE_TEST:GateSelectorCodeDest,0,0,DA_386CGate + DA_DPL3GdtLenequ$-LABEL_GDTGdtPtrdwGdtLen-1;注意，长度都是实际长度减1dd0;段基地址,注意，这里之所以没有直接制定，是因为还没有确定保护模式下gdt的基地址;选择子SelectorDataequLABEL_DESC_DATA-LABEL_GDTSelectorCode16equLABEL_DESC_CODE16-LABEL_GDTSelectorCode32equLABEL_DESC_CODE32-LABEL_GDTSelectorStackequLABEL_DESC_STACK-LABEL_GDT SelectorNormalequLABEL_DESC_NORMAL-LABEL_GDT SelectorLDTequLABEL_DESC_LDT-LABEL_GDT SelectorCodeDest equLABEL_DESC_CODE_DEST -LABEL_GDT SelectorCallGateTest equLABEL_CALL_GATE_TEST -LABEL_GDT +SA_RPL3SelectorVideoequLABEL_DESC_VIDEO-LABEL_GDTSelectorCodeRing3equLABEL_DESC_CODE_RING3 -LABEL_GDT+SA_RPL3 SelectorStack3equLABEL_DESC_STACK3-LABEL_GDT+SA_RPL3 SelectorTSSequLABEL_DESC_TSS-LABEL_GDT ;end of section gdt;--------------------------------------------------------------[SECTION .data][BITS 32]ALIGN 32LABEL_SEG_DATA:SPValueInRealModeldw0PMMessage:db"Coming into protect mode now !",0OffsetPMMessageequPMMessage - $$StrTest:db"ABCDEFGHIJKLMNOPQRSTUVWXYZ",0OffsetStrTestequStrTest - $$SegDataLenequ$ - LABEL_SEG_DATA ;end of section data;-----------------------------section:global stack------------------[SECTION .gs]ALIGN32[BITS 32]LABEL_SEG_STACK:times512db0TopOfStackequ$-LABEL_SEG_STACK-1;----------------------section:LDT----------------------------------[SECTION .ldt]align32[bits32]LABEL_SEG_LDT:LABEL_DESC_CODEA:Descriptor0,CodeALen -1,DA_C+DA_32LDTLenequ$- LABEL_SEG_LDTSelectorCodeA equLABEL_DESC_CODEA - LABEL_SEG_LDT + SA_TIL;end of ldt segment;-------------------------------section:codeA------------------------[section.codeA]align32[bits32]LABEL_SEG_CODEA:movax,SelectorVideomovgs,axmovah,0chmoval,'L'movedi,(2*80+0)*2mov[gs:edi],axjmpSelectorCode16:0CodeALen equ$-LABEL_SEG_CODEA ;end of secion codeA;------------------------------section:s16 begin---------------------[SECTION .s16][BITS 16]LABEL_BEGIN:xchg bx,bxmovax,csmovds,axmoves,axmovss,axmovsp,0100hmov[LABEL_GO_BACK_TO_REAL+3],axmov[SPValueInRealModel],sp;for segment code32xoreax,eaxmoveax,csshleax,4addeax,LABEL_SEG_CODE32;movword[LABEL_DESC_CODE32 +2],axshreax,16movbyte[LABEL_DESC_CODE32 + 4],almovbyte[LABEL_DESC_CODE32 + 7],ah;for segment code16xoreax,eaxmovax,csshleax,4addeax,LABEL_SEG_CODE16;movword[LABEL_DESC_CODE16 +2],axshreax,16movbyte[LABEL_DESC_CODE16 + 4],almovbyte[LABEL_DESC_CODE16 + 7],ah;for segment dataxoreax,eaxmoveax,dsshleax,4addeax,LABEL_SEG_DATA;movword[LABEL_DESC_DATA +2],axshreax,16movbyte[LABEL_DESC_DATA + 4],almovbyte[LABEL_DESC_DATA + 7],ah;for segment ldtxoreax,eaxmoveax,dsshleax,4addeax,LABEL_SEG_LDT;movword[LABEL_DESC_LDT +2],axshreax,16movbyte[LABEL_DESC_LDT + 4],almovbyte[LABEL_DESC_LDT + 7],ah;for segment codeAxoreax,eaxmoveax,dsshleax,4addeax,LABEL_SEG_CODEA;movword[LABEL_DESC_CODEA  +2],axshreax,16movbyte[LABEL_DESC_CODEA + 4],almovbyte[LABEL_DESC_CODEA + 7],ah;for segment stackxoreax,eaxmoveax,dsshleax,4addeax,LABEL_SEG_STACK;movword[LABEL_DESC_STACK +2],axshreax,16movbyte[LABEL_DESC_STACK+ 4],almovbyte[LABEL_DESC_STACK+ 7],ah;no need for video base;for segment dstcodexoreax,eaxmovax,csshleax,4addeax,LABEL_SEG_CODE_DESTmovword[LABEL_DESC_CODE_DEST +2],axshreax,16movbyte[LABEL_DESC_CODE_DEST +4],almovbyte[LABEL_DESC_CODE_DEST +7],ah;for segment code ring 3 xoreax,eaxmovax,csshleax,4addeax,LABEL_SEG_CODE_RING3movword[LABEL_DESC_CODE_RING3 +2],axshreax,16movbyte[LABEL_DESC_CODE_RING3 +4],almovbyte[LABEL_DESC_CODE_RING3 +7],ah;for segment code stack 3 xoreax,eaxmovax,csshleax,4addeax,LABEL_SEG_STACK3 movword[LABEL_DESC_STACK3 +2],axshreax,16movbyte[LABEL_DESC_STACK3 +4],almovbyte[LABEL_DESC_STACK3 +7],ah;for segment code stack 3 xoreax,eaxmovax,csshleax,4addeax,LABEL_SEG_TSSmovword[LABEL_DESC_TSS +2],axshreax,16movbyte[LABEL_DESC_TSS +4],almovbyte[LABEL_DESC_TSS +7],ahxor eax,eaxmovax,dsshleax,4addeax, LABEL_GDTmovdword[GdtPtr +2 ],eaxlgdt[GdtPtr]cliin al,92horal,02hout92h,almoveax,cr0oreax,1movcr0,eaxjmpdword SelectorCode32:0;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;LABEL_REAL_ENTRY:;come here from protect modelmovax,csmovds,axmoves,axmovss,axmovsp,[SPValueInRealModel]inal,92handal,11111101bout92h,alstimovax,4c00hint21h;end of section .s16;------------------------section:code32,start of protect model--------[SECTION .s32][BITS32]LABEL_SEG_CODE32:movax,SelectorVideomovgs,axmovax,SelectorDatamovds,axmovax,SelectorStackmovss,axmovesp,TopOfStackmovah,0chxoresi,esixoredi,edimovesi,OffsetPMMessagemovedi,(80*11+0)*2cld.loopPmMessage:lodsbtestal,aljz.endmov[gs:edi],axaddedi,2jmp.loopPmMessage.end:;callDispReturnmovax,SelectorTSSltraxpushSelectorStack3pushTopOfStack3pushSelectorCodeRing3push0xchgbx,bxretfxchgbx,bxcallSelectorCallGateTest:0;Load LDTmovax,SelectorLDTlldtaxjmpSelectorCodeA:0;function: read and print 8 byte from es:0TestRead:xoresi,esimovecx,8.loopForEightBype:moval,[es:esi]callDispALincesiloop.loopForEightBypecallDispReturnret; be sure of this;funtion:;write 8byte to es:OffsetStrTest;input:esTestWrite:pushesipushedixoresi,esixoredi,edimovesi,OffsetStrTestcld.loopForEightBype:lodsb;ds:si->altestal,aljz.endmov[es:edi],alincedijmp.loopForEightBype.end:popedipopesiret;funtion DispAL;display the number in AL;input: AL-the number;edi-the position to display;modified:ax,ediDispAL:pushecxpushedxmovah,0chmovdl,alshral,4movecx,2.begin:andal,01111bcmpal,9ja.moreThanNineaddal,'0'jmp.end.moreThanNine:subal,0ahaddal,'A'.end:mov[gs:edi],axaddedi,2moval,dlloop.beginaddedi,2popedxpopecxret;DispAL;function DispReturn;if edi=(a*80 + b)*2;then edi=(a*80 + 80)*2DispReturn:pusheaxpushebxmoveax,edimovbl,160divblandeax,0ffh;inceaxmovbl,160mulblmovedi,eaxpopebxpopeaxret;end for function DispReturnSegCode32Lenequ$-LABEL_SEG_CODE32;end of section .s32;---------------------section s16code,before return to real-----------[SECTION .s16code]ALIGN32[BITS 16]LABEL_SEG_CODE16:;return to real modelmovax,SelectorNormal movds,axmoves,axmovfs,axmovgs,axmovss,axmoveax,cr0andal,11111110bmovcr0,eaxLABEL_GO_BACK_TO_REAL:jmp0:LABEL_REAL_ENTRY;Code16Lenequ$-LABEL_SEG_CODE16;end of section s16code;---------------------section:sdest---------------------------------[section.sdest][bits32]LABEL_SEG_CODE_DEST:movax,SelectorVideomovgs,axmovedi,(80*12+0)*2movah,0chmoval,'C'mov[gs:edi],ax;load LDTmovax,SelectorLDTlldtaxjmpSelectorCodeA:0SegCodeDestLenequ$-LABEL_SEG_CODE_DEST;-------------------section stack ring3-----------------------------[section .s3]align 32[bits 32]LABEL_SEG_STACK3:times512db0TopOfStack3equ$-LABEL_SEG_STACK3-1;----------------------section CodeRing3-----------------------------[section .CodeRing3][bits32]LABEL_SEG_CODE_RING3:movax,SelectorVideomovgs,axmovedi,(80*14+0)*2movah,0chmoval,'3'mov[gs:edi],axcallSelectorCallGateTest:0jmp$SegCodeRing3Lenequ$-LABEL_SEG_CODE_RING3;------------------------section TSS-------------------------------[section .tss][bits 32]LABEL_SEG_TSS:dd0ddTopOfStackddSelectorStackdd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dd0dw0dw$-LABEL_SEG_TSS +2db0ffhTSSLenequ$ - LABEL_SEG_TSS