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CWE Rule 194

Unexpected Sign Extension

Since R2023a

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Description

Rule Description

The software performs an operation on a number that causes it to be sign extended when it is transformed into a larger data type. When the original number is negative, this can produce unexpected values that lead to resultant weaknesses.

Polyspace Implementation

The rule checker checks for these issues:

  • Sign change integer conversion overflow

  • Tainted sign change conversion

Examples

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Issue

This issue occurs when converting an unsigned integer to a signed integer. If the variable does not have enough bytes to represent both the original constant and the sign bit, the conversion overflows.

The exact storage allocation for different floating point types depends on your processor. See Target processor type (-target).

Fix

The fix depends on the root cause of the defect. Often the result details (or source code tooltips in Polyspace as You Code) show a sequence of events that led to the defect. You can implement the fix on any event in the sequence. If the result details do not show this event history, you can search for previous references of variables relevant to the defect using right-click options in the source code and find related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface or Interpret Bug Finder Results in Polyspace Access Web Interface (Polyspace Access).

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

Extend Checker

A default Bug Finder analysis might not raise this defect when the input values are unknown and only a subset of inputs cause an issue. To check for defects caused by specific system input values, run a stricter Bug Finder analysis. See Extend Bug Finder Checkers to Find Defects from Specific System Input Values.

Example — Convert from unsigned char to char
char sign_change(void) {
    unsigned char count = 255;

    return (char)count;  //Noncompliant
}

In the return statement, the unsigned character variable count is converted to a signed character. However, char has 8 bits, 1 for the sign of the constant and 7 to represent the number. The conversion operation overflows because 255 uses 8 bits.

Correction — Change conversion types

One possible correction is using a larger integer type. By using an int, there are enough bits to represent the sign and the number value.

int sign_change(void) {
    unsigned char count = 255;

    return (int)count;
}
Issue

This issue occurs when values from unsecure sources are converted, implicitly or explicitly, from signed to unsigned values.

For example, functions that use size_t as arguments implicitly convert the argument to an unsigned integer. Some functions that implicitly convert size_t are:

bcmp
memcpy
memmove
strncmp
strncpy
calloc
malloc
memalign

Risk

If you convert a small negative number to unsigned, the result is a large positive number. The large positive number can create security vulnerabilities. For example, if you use the unsigned value in:

  • Memory size routines — causes allocating memory issues.

  • String manipulation routines — causes buffer overflow.

  • Loop boundaries — causes infinite loops.

Fix

To avoid converting unsigned negative values, check that the value being converted is within an acceptable range. For example, if the value represents a size, validate that the value is not negative and less than the maximum value size.

Extend Checker

By default, Polyspace® assumes that data from external sources are tainted. See Sources of Tainting in a Polyspace Analysis. To consider any data that does not originate in the current scope of Polyspace analysis as tainted, use the command line option -consider-analysis-perimeter-as-trust-boundary.

Example — Set Memory Value with Size Argument
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

enum {
    SIZE10  =  10,
    SIZE100 = 100,
    SIZE128 = 128
};

void bug_taintedsignchange(void) {
    int size;
    scanf("%d",&size);
    char str[SIZE128] = "";
    if (size<SIZE128) {
        memset(str, 'c', size); //Noncompliant
    }
}

In this example, a char buffer is created and filled using memset. The size argument to memset is an input argument to the function.

The call to memset implicitly converts size to unsigned integer. If size is a large negative number, the absolute value could be too large to represent as an integer, causing a buffer overflow.

Correction — Check Value of size

One possible correction is to check if size is inside the valid range. This correction checks if size is greater than zero and less than the buffer size before calling memset.

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

enum {
    SIZE10  =  10,
    SIZE100 = 100,
    SIZE128 = 128
};

void corrected_taintedsignchange(void) {
    int size;
    scanf("%d",&size);
    char str[SIZE128] = "";
    if (size>0 && size<SIZE128) {
        memset(str, 'c', size);  
    }
}

Check Information

Category: Others

Version History

Introduced in R2023a

See Also

Topics

External Websites