CWE-761 释放一个不在缓冲区起始位置的指针

Free of Pointer not at Start of Buffer

结构: Simple

Abstraction: Variant

状态: Incomplete

被利用可能性: unkown

基本描述

The application calls free() on a pointer to a memory resource that was allocated on the heap, but the pointer is not at the start of the buffer.

扩展描述

This can cause the application to crash, or in some cases, modify critical program variables or execute code.

This weakness often occurs when the memory is allocated explicitly on the heap with one of the malloc() family functions and free() is called, but pointer arithmetic has caused the pointer to be in the interior or end of the buffer.

相关缺陷

  • cwe_Nature: ChildOf cwe_CWE_ID: 763 cwe_View_ID: 1000 cwe_Ordinal: Primary

常见的影响

范围 影响 注释
['Integrity', 'Availability', 'Confidentiality'] ['Modify Memory', 'DoS: Crash, Exit, or Restart', 'Execute Unauthorized Code or Commands']

可能的缓解方案

Implementation

策略:

When utilizing pointer arithmetic to traverse a buffer, use a separate variable to track progress through memory and preserve the originally allocated address for later freeing.

Implementation

策略:

When programming in C++, consider using smart pointers provided by the boost library to help correctly and consistently manage memory.

MIT-4.6 Architecture and Design

策略: Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, glibc in Linux provides protection against free of invalid pointers.

Architecture and Design

策略:

Use a language that provides abstractions for memory allocation and deallocation.

Testing

策略:

Use a tool that dynamically detects memory management problems, such as valgrind.

示例代码

例

In this example, the programmer dynamically allocates a buffer to hold a string and then searches for a specific character. After completing the search, the programmer attempts to release the allocated memory and return SUCCESS or FAILURE to the caller. Note: for simplification, this example uses a hard-coded "Search Me!" string and a constant string length of 20.

bad C

#define SUCCESS (1)
#define FAILURE (0)

int contains_char(char c){
char str;
str = (char)malloc(20sizeof(char));
strcpy(str, "Search Me!");
while( str != NULL){
if( str == c ){

/ matched char, free string and return success /
free(str);
return SUCCESS;
}
/ didn't match yet, increment pointer and try next char /

str = str + 1;
}
/ we did not match the char in the string, free mem and return failure */

free(str);
return FAILURE;
}

However, if the character is not at the beginning of the string, or if it is not in the string at all, then the pointer will not be at the start of the buffer when the programmer frees it.

Instead of freeing the pointer in the middle of the buffer, the programmer can use an indexing pointer to step through the memory or abstract the memory calculations by using array indexing.

good C

#define SUCCESS (1)
#define FAILURE (0)

int cointains_char(char c){
char str;
int i = 0;
str = (char)malloc(20sizeof(char));
strcpy(str, "Search Me!");
while( i < strlen(str) ){
if( str[i] == c ){

/ matched char, free string and return success /
free(str);
return SUCCESS;
}
/ didn't match yet, increment pointer and try next char /

i = i + 1;
}
/ we did not match the char in the string, free mem and return failure */

free(str);
return FAILURE;
}

例

This code attempts to tokenize a string and place it into an array using the strsep function, which inserts a \0 byte in place of whitespace or a tab character. After finishing the loop, each string in the AP array points to a location within the input string.

bad C

char ap, argv[10], inputstring;
for (ap = argv; (*ap = strsep(&inputstring, " \t")) != NULL;)
if (ap != '\0')
if (++ap >= &argv[10])
break;

/.../
free(ap[4]);

Since strsep is not allocating any new memory, freeing an element in the middle of the array is equivalent to free a pointer in the middle of inputstring.

例

Consider the following code in the context of a parsing application to extract commands out of user data. The intent is to parse each command and add it to a queue of commands to be executed, discarding each malformed entry.

bad C


//hardcode input length for simplicity
char input = (char) malloc(40sizeof(char));
char tok;
char sep = " \t";

get_user_input( input );

/ The following loop will parse and process each token in the input string /

tok = strtok( input, sep);
while( NULL != tok ){
if( isMalformed( tok ) ){

/ ignore and discard bad data */
free( tok );
}
else{
add_to_command_queue( tok );
}
tok = strtok( NULL, sep));
}

While the above code attempts to free memory associated with bad commands, since the memory was all allocated in one chunk, it must all be freed together.

One way to fix this problem would be to copy the commands into a new memory location before placing them in the queue. Then, after all commands have been processed, the memory can safely be freed.

good C


//hardcode input length for simplicity
char input = (char) malloc(40sizeof(char));
char tok, command;
char sep = " \t";

get_user_input( input );

/ The following loop will parse and process each token in the input string /

tok = strtok( input, sep);
while( NULL != tok ){
if( !isMalformed( command ) ){

/ copy and enqueue good data /
command = (char*) malloc( (strlen(tok) + 1) * sizeof(char) );
strcpy( command, tok );
add_to_command_queue( command );
}
tok = strtok( NULL, sep));
}

free( input )

Notes

分类映射

映射的分类名 ImNode ID Fit Mapped Node Name
Software Fault Patterns SFP12 Faulty Memory Release

引用