|
| 1 | +# ARR39-C: Do not add or subtract a scaled integer to a pointer |
| 2 | + |
| 3 | +This query implements the CERT-C rule ARR39-C: |
| 4 | + |
| 5 | +> Do not add or subtract a scaled integer to a pointer |
| 6 | +
|
| 7 | + |
| 8 | +## Description |
| 9 | + |
| 10 | +Pointer arithmetic is appropriate only when the pointer argument refers to an array (see [ARR37-C. Do not add or subtract an integer to a pointer to a non-array object](https://wiki.sei.cmu.edu/confluence/display/c/ARR37-C.+Do+not+add+or+subtract+an+integer+to+a+pointer+to+a+non-array+object)), including an array of bytes. When performing pointer arithmetic, the size of the value to add to or subtract from a pointer is automatically scaled to the size of the type of the referenced array object. Adding or subtracting a scaled integer value to or from a pointer is invalid because it may yield a pointer that does not point to an element within or one past the end of the array. (See [ARR30-C. Do not form or use out-of-bounds pointers or array subscripts](https://wiki.sei.cmu.edu/confluence/display/c/ARR30-C.+Do+not+form+or+use+out-of-bounds+pointers+or+array+subscripts).) |
| 11 | + |
| 12 | +Adding a pointer to an array of a type other than character to the result of the `sizeof` operator or `offsetof` macro, which returns a size and an offset, respectively, violates this rule. However, adding an array pointer to the number of array elements, for example, by using the `arr[sizeof(arr)/sizeof(arr[0])])` idiom, is allowed provided that `arr` refers to an array and not a pointer. |
| 13 | + |
| 14 | +## Noncompliant Code Example |
| 15 | + |
| 16 | +In this noncompliant code example, `sizeof(buf)` is added to the array `buf`. This example is noncompliant because `sizeof(buf)` is scaled by `int` and then scaled again when added to `buf`. |
| 17 | + |
| 18 | +```cpp |
| 19 | +enum { INTBUFSIZE = 80 }; |
| 20 | + |
| 21 | +extern int getdata(void); |
| 22 | +int buf[INTBUFSIZE]; |
| 23 | + |
| 24 | +void func(void) { |
| 25 | + int *buf_ptr = buf; |
| 26 | + |
| 27 | + while (buf_ptr < (buf + sizeof(buf))) { |
| 28 | + *buf_ptr++ = getdata(); |
| 29 | + } |
| 30 | +} |
| 31 | +``` |
| 32 | + |
| 33 | +## Compliant Solution |
| 34 | + |
| 35 | +This compliant solution uses an unscaled integer to obtain a pointer to the end of the array: |
| 36 | + |
| 37 | +```cpp |
| 38 | +enum { INTBUFSIZE = 80 }; |
| 39 | + |
| 40 | +extern int getdata(void); |
| 41 | +int buf[INTBUFSIZE]; |
| 42 | + |
| 43 | +void func(void) { |
| 44 | + int *buf_ptr = buf; |
| 45 | + |
| 46 | + while (buf_ptr < (buf + INTBUFSIZE)) { |
| 47 | + *buf_ptr++ = getdata(); |
| 48 | + } |
| 49 | +} |
| 50 | +``` |
| 51 | + |
| 52 | +## Noncompliant Code Example |
| 53 | + |
| 54 | +In this noncompliant code example, `skip` is added to the pointer `s`. However, `skip` represents the byte offset of `ull_b` in `struct big`. When added to `s`, `skip` is scaled by the size of `struct big`. |
| 55 | + |
| 56 | +```cpp |
| 57 | +#include <string.h> |
| 58 | +#include <stdlib.h> |
| 59 | +#include <stddef.h> |
| 60 | + |
| 61 | +struct big { |
| 62 | + unsigned long long ull_a; |
| 63 | + unsigned long long ull_b; |
| 64 | + unsigned long long ull_c; |
| 65 | + int si_e; |
| 66 | + int si_f; |
| 67 | +}; |
| 68 | + |
| 69 | +void func(void) { |
| 70 | + size_t skip = offsetof(struct big, ull_b); |
| 71 | + struct big *s = (struct big *)malloc(sizeof(struct big)); |
| 72 | + if (s == NULL) { |
| 73 | + /* Handle malloc() error */ |
| 74 | + } |
| 75 | + |
| 76 | + memset(s + skip, 0, sizeof(struct big) - skip); |
| 77 | + /* ... */ |
| 78 | + free(s); |
| 79 | + s = NULL; |
| 80 | +} |
| 81 | +``` |
| 82 | +
|
| 83 | +## Compliant Solution |
| 84 | +
|
| 85 | +This compliant solution uses an `unsigned char *` to calculate the offset instead of using a `struct big *`, which would result in scaled arithmetic: |
| 86 | +
|
| 87 | +```cpp |
| 88 | +#include <string.h> |
| 89 | +#include <stdlib.h> |
| 90 | +#include <stddef.h> |
| 91 | + |
| 92 | +struct big { |
| 93 | + unsigned long long ull_a; |
| 94 | + unsigned long long ull_b; |
| 95 | + unsigned long long ull_c; |
| 96 | + int si_d; |
| 97 | + int si_e; |
| 98 | +}; |
| 99 | +
|
| 100 | +void func(void) { |
| 101 | + size_t skip = offsetof(struct big, ull_b); |
| 102 | + unsigned char *ptr = (unsigned char *)malloc( |
| 103 | + sizeof(struct big) |
| 104 | + ); |
| 105 | + if (ptr == NULL) { |
| 106 | + /* Handle malloc() error */ |
| 107 | + } |
| 108 | +
|
| 109 | + memset(ptr + skip, 0, sizeof(struct big) - skip); |
| 110 | + /* ... */ |
| 111 | + free(ptr); |
| 112 | + ptr = NULL; |
| 113 | +} |
| 114 | +``` |
| 115 | + |
| 116 | +## Noncompliant Code Example |
| 117 | + |
| 118 | +In this noncompliant code example, `wcslen(error_msg) * sizeof(wchar_t)` bytes are scaled by the size of `wchar_t` when added to `error_msg`: |
| 119 | + |
| 120 | +```cpp |
| 121 | +#include <wchar.h> |
| 122 | +#include <stdio.h> |
| 123 | + |
| 124 | +enum { WCHAR_BUF = 128 }; |
| 125 | + |
| 126 | +void func(void) { |
| 127 | + wchar_t error_msg[WCHAR_BUF]; |
| 128 | + |
| 129 | + wcscpy(error_msg, L"Error: "); |
| 130 | + fgetws(error_msg + wcslen(error_msg) * sizeof(wchar_t), |
| 131 | + WCHAR_BUF - 7, stdin); |
| 132 | + /* ... */ |
| 133 | +} |
| 134 | +``` |
| 135 | + |
| 136 | +## Compliant Solution |
| 137 | + |
| 138 | +This compliant solution does not scale the length of the string; `wcslen()` returns the number of characters and the addition to `error_msg` is scaled: |
| 139 | + |
| 140 | +```cpp |
| 141 | +#include <wchar.h> |
| 142 | +#include <stdio.h> |
| 143 | + |
| 144 | +enum { WCHAR_BUF = 128 }; |
| 145 | +const wchar_t ERROR_PREFIX[7] = L"Error: "; |
| 146 | + |
| 147 | +void func(void) { |
| 148 | + const size_t prefix_len = wcslen(ERROR_PREFIX); |
| 149 | + wchar_t error_msg[WCHAR_BUF]; |
| 150 | + |
| 151 | + wcscpy(error_msg, ERROR_PREFIX); |
| 152 | + fgetws(error_msg + prefix_len, |
| 153 | + WCHAR_BUF - prefix_len, stdin); |
| 154 | + /* ... */ |
| 155 | +} |
| 156 | +``` |
| 157 | + |
| 158 | +## Risk Assessment |
| 159 | + |
| 160 | +Failure to understand and properly use pointer arithmetic can allow an attacker to execute arbitrary code. |
| 161 | + |
| 162 | +<table> <tbody> <tr> <th> Rule </th> <th> Severity </th> <th> Likelihood </th> <th> Remediation Cost </th> <th> Priority </th> <th> Level </th> </tr> <tr> <td> ARR39-C </td> <td> High </td> <td> Probable </td> <td> High </td> <td> <strong>P6</strong> </td> <td> <strong>L2</strong> </td> </tr> </tbody> </table> |
| 163 | + |
| 164 | + |
| 165 | +## Automated Detection |
| 166 | + |
| 167 | +<table> <tbody> <tr> <th> Tool </th> <th> Version </th> <th> Checker </th> <th> Description </th> </tr> <tr> <td> <a> Astrée </a> </td> <td> 22.04 </td> <td> <strong>scaled-pointer-arithmetic </strong> </td> <td> Partially checked Besides direct rule violations, Astrée reports all (resulting) out-of-bound array accesses. </td> </tr> <tr> <td> <a> Axivion Bauhaus Suite </a> </td> <td> 7.2.0 </td> <td> <strong>CertC-ARR39</strong> </td> <td> Fully implemented </td> </tr> <tr> <td> <a> CodeSonar </a> </td> <td> 7.0p0 </td> <td> <strong>LANG.MEM.BO</strong> <strong>LANG.MEM.BU</strong> <strong>LANG.MEM.TBA</strong> <strong>LANG.MEM.TO</strong> <strong>LANG.MEM.TULANG.STRUCT.PARITH</strong> <strong>LANG.STRUCT.PBB</strong> <strong>LANG.STRUCT.PPE</strong> <strong>BADFUNC.BO.\*</strong> </td> <td> Buffer overrun Buffer underrun Tainted buffer access Type overrun Type underrun Pointer Arithmetic Pointer before beginning of object Pointer past end of object A collection of warning classes that report uses of library functions prone to internal buffer overflows. </td> </tr> <tr> <td> <a> Coverity </a> </td> <td> 2017.07 </td> <td> <strong>BAD_SIZEOF</strong> </td> <td> Partially implemented </td> </tr> <tr> <td> <a> Helix QAC </a> </td> <td> 2022.2 </td> <td> <strong>C4955, C4956, C4957</strong> <strong>C++4955, C++4956, C++4957</strong> </td> <td> </td> </tr> <tr> <td> <a> Klocwork </a> </td> <td> 2022.2 </td> <td> <strong>MISRA.PTR.ARITH.2012</strong> </td> <td> </td> </tr> <tr> <td> <a> LDRA tool suite </a> </td> <td> 9.7.1 </td> <td> <strong>47 S, 489 S, 567 S,64 X, 66 X, 68 X,69 X, 70 X, 71 X</strong> </td> <td> Partially implemented </td> </tr> <tr> <td> <a> Parasoft C/C++test </a> </td> <td> 2022.1 </td> <td> <strong>CERT_C-ARR39-a</strong> <strong>CERT_C-ARR39-b</strong> <strong>CERT_C-ARR39-c</strong> </td> <td> Avoid accessing arrays out of bounds Pointer arithmetic should not be used Do not add or subtract a scaled integer to a pointer </td> </tr> <tr> <td> Polyspace Bug Finder </td> <td> R2022a </td> <td> <a> CERT C: Rule ARR39-C </a> </td> <td> Checks for: Incorrect pointer scalingncorrect pointer scaling, pointer access out of boundsointer access out of bounds, possible misuse of sizeofossible misuse of sizeof. Rule partially covered. </td> </tr> <tr> <td> <a> PRQA QA-C </a> </td> <td> 9.7 </td> <td> <strong> 4955, 4956, 4957</strong> </td> <td> </td> </tr> <tr> <td> <a> PRQA QA-C++ </a> </td> <td> 4.4 </td> <td> <strong>4955, 4956, 4957</strong> </td> <td> </td> </tr> <tr> <td> <a> RuleChecker </a> </td> <td> 22.04 </td> <td> <strong>scaled-pointer-arithmetic</strong> </td> <td> Partially checked </td> </tr> <tr> <td> <a> TrustInSoft Analyzer </a> </td> <td> 1.38 </td> <td> <strong>index_in_address</strong> </td> <td> Exhaustively detects undefined behavior (see <a> one compliant and one non-compliant example </a> ). </td> </tr> </tbody> </table> |
| 168 | + |
| 169 | + |
| 170 | +## Related Vulnerabilities |
| 171 | + |
| 172 | +Search for [vulnerabilities](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-vulnerability) resulting from the violation of this rule on the [CERT website](https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+EXP41-C). |
| 173 | + |
| 174 | +## Related Guidelines |
| 175 | + |
| 176 | +[Key here](https://wiki.sei.cmu.edu/confluence/display/c/How+this+Coding+Standard+is+Organized#HowthisCodingStandardisOrganized-RelatedGuidelines) (explains table format and definitions) |
| 177 | + |
| 178 | +<table> <tbody> <tr> <th> Taxonomy </th> <th> Taxonomy item </th> <th> Relationship </th> </tr> <tr> <td> <a> CERT C Secure Coding Standard </a> </td> <td> <a> ARR30-C. Do not form or use out-of-bounds pointers or array subscripts </a> </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> CERT C Secure Coding Standard </a> </td> <td> <a> ARR37-C. Do not add or subtract an integer to a pointer to a non-array object </a> </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> ISO/IEC TR 24772:2013 </a> </td> <td> Pointer Casting and Pointer Type Changes \[HFC\] </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> ISO/IEC TR 24772:2013 </a> </td> <td> Pointer Arithmetic \[RVG\] </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> MISRA C:2012 </a> </td> <td> Rule 18.1 (required) </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> MISRA C:2012 </a> </td> <td> Rule 18.2 (required) </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> MISRA C:2012 </a> </td> <td> Rule 18.3 (required) </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> MISRA C:2012 </a> </td> <td> Rule 18.4 (advisory) </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> CWE 2.11 </a> </td> <td> <a> CWE-468 </a> , Incorrect Pointer Scaling </td> <td> 2017-07-07: CERT: Exact </td> </tr> </tbody> </table> |
| 179 | + |
| 180 | + |
| 181 | +## Bibliography |
| 182 | + |
| 183 | +<table> <tbody> <tr> <td> \[ <a> Dowd 2006 </a> \] </td> <td> Chapter 6, "C Language Issues" </td> </tr> <tr> <td> \[ <a> Murenin 07 </a> \] </td> <td> </td> </tr> </tbody> </table> |
| 184 | + |
| 185 | + |
| 186 | +## Implementation notes |
| 187 | + |
| 188 | +None |
| 189 | + |
| 190 | +## References |
| 191 | + |
| 192 | +* CERT-C: [ARR39-C: Do not add or subtract a scaled integer to a pointer](https://wiki.sei.cmu.edu/confluence/display/c) |
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