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There are many possible ways of dealing with buffer overflow bugs. The first and most obvious way is of course to try to avoid them in the first place, but that doesn t always prove to be as simple as it seems. Sure, it would take a really careless developer to put something like our poor launch in a production system,
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but there are other, far more subtle mistakes that can create potential buffer overflow bugs. One technique that aims to automatically prevent these problems from occurring is by the use of automatic, compiler-generated stack checking. The idea is quite simple: For any function that accesses local variables by reference, push an extra cookie or canary to the stack between the last local variable and the function s return address. This cookie should then be validated before the function returns to the caller. If the cookie has been modified, program execution immediately stops. This ensures that the return value hasn t been overwritten with some other address and prevents the execution of any kind of malicious code. One thing that s immediately clear about this approach is that the cookie must be a random number. If it s not, an attacker could simply add the cookie s value as part of the overflowing payload and bypass the stack protection. The solution is to use a pseudorandom number as a cookie. If you re wondering just how random pseudorandom numbers can be, take a look at [Knuth2] Donald E. Knuth. The Art of Computer Programming Volume 2: Seminumerical Algorithms (Second Edition). Addison Wesley, but suffice it to say that they re random enough for this purpose. With a pseudorandom number, the attacker has no way of knowing in advance what the cookie is going to be, and so it becomes impossible to fool the cookie verification code (though it s still possible to work around this whole mechanism in other ways, as explained later in this chapter). The following code is the same launch function from before, except that stack checking has been added (using the /GS option in the Microsoft C/C++ compiler).
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7!launch: 00401060 sub 00401063 mov 00401068 mov 0040106c mov 00401070 lea 00401073 sub 00401075 mov 00401077 mov 0040107a inc 0040107b test 0040107d jnz 0040107f push 00401080 lea 00401084 dec 00401085 mov 00401088 inc 00401089 test 0040108b jnz 0040108d mov 00401092 mov esp,0x68 eax,[7!__security_cookie (0040a428)] [esp+0x64],eax eax,[esp+0x6c] edx,[esp] edx,eax cl,[eax] [edx+eax],cl eax cl,cl 7!launch+0x15 (00401075) edi edi,[esp+0x4] edi al,[edi+0x1] edi al,al 7!launch+0x25 (00401085) eax,[7!'string (00408128)] cl,[7!'string +0x4 (0040812c)]
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00401098 0040109c 0040109e 0040109f 004010a2 004010a7 004010ab 004010ae 004010af 004010b4 004010b7 lea mov push mov call mov add pop call add ret edx,[esp+0x4] [edi],eax edx [edi+0x4],cl 7!system (00401110) ecx,[esp+0x6c] esp,0x4 edi 7!__security_check_cookie (004011d7) esp,0x68
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The __security_check_cookie function is called before launch returns in order to verify that the cookie has not been corrupted. Here is what __security_check_cookie does.
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__security_check_cookie: 004011d7 cmp ecx,[7!__security_cookie (0040a428)] 004011dd jnz 7!__security_check_cookie+0x9 (004011e0) 004011df ret 004011e0 jmp 7!report_failure (004011a6)
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This idea was originally presented in [Cowan], Crispin Cowan, Calton Pu, David Maier, Heather Hinton, Peat Bakke, Steve Beattie, Aaron Grier, Perry Wagle, and Qian Zhang. Automatic Detection and Prevention of Buffer-Overflow Attacks. The 7th USENIX Security Symposium. San Antonio, TX, January 1998 and has since been implemented in several compilers. The latest versions of the Microsoft C/C++ compilers support stack checking, and the Microsoft operating systems (starting with Windows Server 2003 and Windows XP Service Pack 2) take advantage of this feature. In Windows, the cookie is stored in a global variable within the protected module (usually in __security_cookie). This variable is initialized by __security_init_cookie when the module is loaded, and is randomized based on the current process and thread IDs, along with the current time or the value of the hardware performance counter (see Listing 7.1). In case you re wondering, here is the source code for __security_init_cookie. This code is embedded into any program built using the Microsoft compiler that has stack checking enabled.
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void __cdecl __security_init_cookie(void) { DWORD_PTR cookie; FT systime; LARGE_INTEGER perfctr;
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Listing 7.1 The __security_init_cookie function that initializes the stack-checking cookie in code generated by the Microsoft C/C++ compiler. (continued)
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