Buffer Over ow in Java

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6 Buffer Over ow
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The Untold Impact of Dynamic Allocation
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The memory allocation approach you choose will affect how you use static analysis to identify potential buffer over ows and other memory violations in your code Static buffer allocation schemes are typically easier to understand and verify, for both humans and static analysis tools Because buffers are allocated with a xed size at compile time, tools can more easily reason about the size of buffers used in various operations and are likely to identify errors more accurately Conversely, a dynamic buffer allocation scheme could introduce false positives where operations that are agged as dangerous are actually made safe due to runtime constraints Dynamic memory allocation can also introduce a variety of errors that are not present with static allocation, including memory leaks, use-after-free errors, and double free errors These errors can be difficult to track down manually because they typically involve complex control flow paths and lend themselves to detection with model checking Although memory leaks deplete available resources, use-after-free and double free errors are often impactful because they can cause segmentation faults and potentially introduce buffer over ow vulnerabilities Use-after-free and double free errors have two common and sometimes overlapping causes: Error conditions and other exceptional circumstances Confusion over which part of the program is responsible for freeing the memory Use-after-free errors occur when a program continues to use a pointer after it has been freed If the freed memory is reallocated, an attacker can use a pointer to the memory to launch a buffer over ow attack The code in Example 66 illustrates a useafter-free error
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Example 66 A use-after-free vulnerability in simple error-handling code
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char* ptr = (char*)malloc (SIZE); if (tryOperation() == OPERATION_FAILED) { free(ptr); errors++; }
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Introduction to Buffer Over ow
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if (errors > 0) { logError("operation aborted before commit", ptr); }
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Double free errors occur when free() is called more than once with the same memory address as an argument without an intervening allocation of the memory at that address When a program calls free() twice with the same argument, the program s memory management data structures can become corrupted This corruption can cause the program to crash or, in some circumstances, cause two later calls to malloc() to return the same pointer If malloc() returns the same value twice and the program later gives the attacker control over the data that are written into this doubly-allocated memory, the program becomes vulnerable to a buffer overflow attack Example 67 illustrates a double free error
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Example 67 A double free vulnerability in simple error-handling code
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char* ptr = (char*)malloc (SIZE); if (tryOperation() == OPERATION_FAILED) { free(ptr); errors++; } free(ptr);
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You can get some protection against these kinds of vulnerabilities by giving up all references to the memory immediately after calling free()This is commonly achieved by replacing calls to free() with a macro that assigns null to the pointer immediately after it is freed, which might be de ned as follows:
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#define FREE( ptr ) {free(ptr); ptr = NULL;}
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Note that this approach will not help if the variable in question is aliased and freed multiple times using different aliases
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6 Buffer Over ow
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Static Analysis: Enforcing Null-After-Free
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You can enforce a policy that requires null be assigned to pointers after they are freed using model checking The following rule looks for calls to free() and then reports an error if the next operation does not set the freed variable to NULL
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Model checking rule:
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start (other operations)
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free(x) freed
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error
x = NULL; (other operations) nulled
Tracking Buffer Sizes Beyond simple string lengths and stack-allocated variables, C and C++ give programmers little help tracking the size of buffers The sizeof operator returns the length of memory allocated for a variable if it s allocated on the stack in the current scope, but it returns only the size of the pointer if the variable is allocated on the heap If a buffer contains a string, the size of the memory the string occupies can be computed by counting the bytes in the buffer before a null terminator is found The only general solution is to explicitly track the current size of each buffer as separate value This enables you to avoid relying on assumptions to ensure that operations on the buffer are safe, but it requires that the length stored for the buffer be updated whenever an operation is performed that alters its size This section outlines some common strategies for tracking buffer sizes and shows a piece of code that is retro tted to include explicit size information The most common approach to tracking a buffer and its size is to store them together in a composite data structure (such as a struct or class) This approach is akin to a rudimentary approximation of memory safety