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The code in Example 1114 will behave correctly if it is executed verbatim, but if the code is compiled using some optimizing compilers, such as Microsoft Visual C++(R) NET and older versions of GCC 3x, the call to memset() will be removed as a dead store because the buffer pwd is not subsequently used [Howard and LeBlanc, 2002] Because the buffer pwd contains a sensitive value, the application might be vulnerable to attack if the data are left memory resident If attackers are able to access the correct region of memory, they could use the recovered password to gain control of the system The problem here is that many compilers and many programming languages do not take this and other security concerns into consideration in their efforts to improve ef ciency Optimizing compilers are a boon to performance, so disabling optimization is rarely a reasonable option The solution is to communicate to the compiler exactly how the program should behave Because support for this communication is imperfect and varies from platform to platform, current solutions to the problem are imperfect as well Current versions of GCC, such as 344 and 412, treat zero differently from other values when it is passed to memset() and do not optimize out calls to memset( ,0, ), but this behavior is neither advertised as a security feature or made an explicit part of the language through a separate function On compilers without this specialized behavior, it is often possible to force the compiler into retaining calls to scrubbing functions by using the variable in a function that has a simple external effect, such as printing its value, after it is cleaned in memory Another option involves volatile pointers, which are not currently optimized because they can be modified from outside the application You can make use of this fact to trick the compiler by casting pointers to sensitive data to volatile pointers, as shown in Example 1115
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Example 1115 Prevent memset() calls from being optimized using volatile pointers
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void GetData(char *MFAddr) { char pwd[64]; if (GetPasswordFromUser(pwd, sizeof(pwd))) { if (ConnectToMainframe(MFAddr, pwd)) { // Interaction with mainframe } } memset(pwd, 0, sizeof(pwd)); *(volatile char*)pwd = *(volatile char*)pwd; }
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The risk in relying on tricks, whether they are built into the compiler or the code itself, is that they rely on the current compiler behavior, which will continue to evolve in the future As compiler technology changes, security aws such as this one could be reintroduced even if an application s source code has remained unchanged Microsoft has taken the best approach thus far by adding a function specifically designed to erase memory securely On recent Microsoft platforms, use SecureZeroMemory(), which is a security-enhanced API that replaces memset() and ZeroMemory() [Howard, 2002], and is guaranteed to erase memory without the risk of being optimized out Although the current implementation of SecureZeroMemory() uses the volatile pointer trick, because its protection from optimization is part of the contract it offers users, it is protected from future changes in compiler behavior If compiler behavior does change in such a way that invalidates the current implementation, SecureZeroMemory() will be updated to continue to provide the same functionality in light of the new compiler behavior
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Use static analysis to identify uses of memset() and other security-relevant operations that an optimizing compiler might remove The tool should not flag all calls to these functions only calls in which the memory being scrubbed is not referenced after the call
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Model checking rule:
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memset (x,0) zeroed reference to x x is unreachable error
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Prevent Unnecessary Duplication of Secrets As with any secret, the more times it s repeated, the harder it is to keep Speaking broadly, secrets in software can be duplicated in two ways: explicitly and implicitly Actively minimize the number of explicit copies of each
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