A basic check that ensures unsigned addition will not over ow in Java

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Example 77 A basic check that ensures unsigned addition will not over ow
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if (A > MAX_INT B) handle_error(); return A + B;
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The tests in Example 78 demonstrate that preventing over ow in signed integer addition is more complicated and requires enumerating different cases, depending on sign of the operands
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Example 78 A more complex check that ensures signed addition will not over ow
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if(!((rhs ^ lhs) < 0)) //test for +/- combo { //either two negatives, or 2 positives if(rhs < 0) { //two negatives if(lhs < INT_MIN - rhs) //remember rhs < 0 { handle_error() } //ok } else {
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Continues
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//two positives if(INT_MAX - lhs < rhs) { handle_error() } //OK } } //else overflow not possible return lhs + rhs;
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7 Bride of Buffer Over ow
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Static Analysis: Beware Integral User Input
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Use static analysis to identify integer over ow vulnerabilities by agging places where user-controlled integral values are passed to memory-allocation operations For example, to identify the integer over ow error in the OpenSSH code in Example 71, use the following rules:
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Source rule:
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Function: packet_get_int() Postcondition: return value is tainted
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Sink rule:
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Function: xmalloc() Precondition: argument must not be tainted Severity: medium
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If user input is used in arithmetic statements, the tool should produce a more severe warning because the likelihood that the input will cause an integer over ow is greater To increase the severity of the warning reported in Example 71, add a special taint ag to user input that is multiplied by another value with the following rule:
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Pass-through rule:
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Function: multiplication operator Postcondition: result is tainted with MULT if either the multiplier or the multiplicand is tainted
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Then, in another sink rule for xmalloc(), check for the presence of the arithmetic taint ag and report a higher-severity warning if it is present:
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Function: xmalloc() Precondition: argument must not be tainted with MULT Severity: high
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Runtime Protection
Runtime Protection
The best way to avoid buffer over ow vulnerabilities is to use a language that does not allow them to occur Java and C# are two such languages If you have a large body of existing code, switching languages is not usually an option In the last few years, two university research projects have produced safer C dialects Such dialects make it possible to gracefully transition to a safer foundation without having to entirely rewrite the program Compensating mechanisms that are applied after the code is written are even easier to implement The down side is that these mechanisms cannot promise the same level of protection In other words, they don t really solve the problem Safer Programming Languages Despite the frequent appearance of public exploits that might indicate otherwise, a very good solution to the buffer over ow problem already exists Java, C#, Python, Ruby, and a variety of other programming languages virtually eliminate the possibility of buffer over ow errors4 However, the protections against buffer over ow that these languages provide come at a cost This section outlines the unavoidable trade-offs between safety and other desirable language properties, such as performance and exibility No single right answer exists for every program; we strive to give you the information necessary to make an informed decision We use the term safe to refer to languages that automatically perform runtime checks to prevent programs from violating the bounds of allocated memory Safe languages must provide two properties to ensure that programs respect allocation bounds: memory safety and type safety Memory safety is the real goal it means that the program will not read or write data outside the bounds of allocated regions To achieve memory safety, a language must also enforce type safety so that it can keep track of the memory allocation bounds Without type safety, any arbitrary value could be used as a reference into memory Beyond the possibility of buffer over ow, unsafe languages, such as C and C++, value compile-time optimization and concise expression over
4 Because many of these systems are implemented in C, it is still possible for buffer over ow vulnerabilities to occur in the underlying implementation For example, a buffer over ow vulnerability was recently identi ed in the kjs JavaScript interpreter used throughout KDE, including Konqueror The vulnerability allows specially crafted JavaScript to cause a heap over ow and execute malicious code on a remote machine [Mueller, 2006]