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Copying data to a buffer that is not large enough to hold that data results in a buffer overflow. Buffer overflows occur frequently when manipulating strings [Seacord 2013b]. To prevent such errors, either limit copies through truncation or, preferably, ensure that the destination is of sufficient size to hold the character data to be copied and the null-termination character. (See STR03-C. Do not inadvertently truncate a string.)

When strings live on the heap, this rule is a specific instance of MEM35-C. Allocate sufficient memory for an object. Because strings are represented as arrays of characters, this rule is related to both ARR30-C. Do not form or use out-of-bounds pointers or array subscripts and ARR38-C. Guarantee that library functions do not form invalid pointers.

Noncompliant Code Example (Off-by-One Error)

This noncompliant code example demonstrates an off-by-one error [Dowd 2006]. The loop copies data from src to dest. However, because the loop does not account for the null-termination character, it may be incorrectly written 1 byte past the end of dest.

#include <stddef.h>
 
void copy(size_t n, char src[n], char dest[n]) {
   size_t i;
 
   for (i = 0; src[i] && (i < n); ++i) {
     dest[i] = src[i];
   }
   dest[i] = '\0';
}

Compliant Solution (Off-by-One Error)

In this compliant solution, the loop termination condition is modified to account for the null-termination character that is appended to dest:

#include <stddef.h>
 
void copy(size_t n, char src[n], char dest[n]) {
   size_t i;
 
   for (i = 0; src[i] && (i < n - 1); ++i) {
     dest[i] = src[i];
   }
   dest[i] = '\0';
}

Noncompliant Code Example (gets())

The gets() function, which was deprecated in the C99 Technical Corrigendum 3 and removed from C11, is inherently unsafe and should never be used because it provides no way to control how much data is read into a buffer from stdin. This noncompliant code example assumes that gets() will not read more than BUFFER_SIZE - 1 characters from stdin. This is an invalid assumption, and the resulting operation can result in a buffer overflow.

The gets() function reads characters from the stdin into a destination array until end-of-file is encountered or a newline character is read. Any newline character is discarded, and a null character is written immediately after the last character read into the array.

#include <stdio.h>
 
#define BUFFER_SIZE 1024

void func(void) {
  char buf[BUFFER_SIZE];
  if (gets(buf) == NULL) {
    /* Handle error */
  }
}

See also MSC24-C. Do not use deprecated or obsolescent functions.

Compliant Solution (fgets())

The fgets() function reads, at most, one less than the specified number of characters from a stream into an array. This solution is compliant because the number of characters copied from stdin to buf cannot exceed the allocated memory:

#include <stdio.h>
#include <string.h>
 
enum { BUFFERSIZE = 32 };
 
void func(void) {
  char buf[BUFFERSIZE];
  int ch;

  if (fgets(buf, sizeof(buf), stdin)) {
    /* fgets() succeeded; scan for newline character */
    char *p = strchr(buf, '\n');
    if (p) {
      *p = '\0';
    } else {
      /* Newline not found; flush stdin to end of line */
      while ((ch = getchar()) != '\n' && ch != EOF)
        ;
      if (ch == EOF && !feof(stdin) && !ferror(stdin)) {
          /* Character resembles EOF; handle error */ 
      }
    }
  } else {
    /* fgets() failed; handle error */
  }
}

The fgets() function is not a strict replacement for the gets() function because fgets() retains the newline character (if read) and may also return a partial line. It is possible to use fgets() to safely process input lines too long to store in the destination array, but this is not recommended for performance reasons. Consider using one of the following compliant solutions when replacing gets().

Compliant Solution (gets_s())

The gets_s() function reads, at most, one less than the number of characters specified from the stream pointed to by stdin into an array.

The C Standard, Annex K [ISO/IEC 9899:2011], states

No additional characters are read after a new-line character (which is discarded) or after end-of-file. The discarded new-line character does not count towards number of characters read. A null character is written immediately after the last character read into the array.

If end-of-file is encountered and no characters have been read into the destination array, or if a read error occurs during the operation, then the first character in the destination array is set to the null character and the other elements of the array take unspecified values:

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
 
void func(void) {
  char buf[BUFFERSIZE];

  if (gets_s(buf, sizeof(buf)) == NULL) {
    /* Handle error */
  }
}

Compliant Solution (getline(), POSIX)

The getline() function is similar to the fgets() function but can dynamically allocate memory for the input buffer. If passed a null pointer, getline() dynamically allocates a buffer of sufficient size to hold the input. If passed a pointer to dynamically allocated storage that is too small to hold the contents of the string, the getline() function resizes the buffer, using realloc(), rather than truncating the input. If successful, the getline() function returns the number of characters read, which can be used to determine if the input has any null characters before the newline. The getline() function works only with dynamically allocated buffers. Allocated memory must be explicitly deallocated by the caller to avoid memory leaks. (See MEM31-C. Free dynamically allocated memory when no longer needed.)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
void func(void) {
  int ch;
  size_t buffer_size = 32;
  char *buffer = malloc(buffer_size);
 
  if (!buffer) {
    /* Handle error */
    return;
  }

  if ((ssize_t size = getline(&buffer, &buffer_size, stdin))
        == -1) {
    /* Handle error */
  } else {
    char *p = strchr(buffer, '\n');
    if (p) {
      *p = '\0';
    } else {
      /* Newline not found; flush stdin to end of line */
      while ((ch = getchar()) != '\n' && ch != EOF)
	    ;
	  if (ch == EOF && !feof(stdin) && !ferror(stdin)) {
         /* Character resembles EOF; handle error */
      }
    }
  }
  free (buffer);
}

Note that the getline() function uses an in-band error indicator, in violation of ERR02-C. Avoid in-band error indicators.

Noncompliant Code Example (getchar())

Reading one character at a time provides more flexibility in controlling behavior, though with additional performance overhead. This noncompliant code example uses the getchar() function to read one character at a time from stdin instead of reading the entire line at once. The stdin stream is read until end-of-file is encountered or a newline character is read. Any newline character is discarded, and a null character is written immediately after the last character read into the array. Similar to the noncompliant code example that invokes gets(), there are no guarantees that this code will not result in a buffer overflow.

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
 
void func(void) {
  char buf[BUFFERSIZE];
  char *p;
  int ch;
  p = buf;
  while ((ch = getchar()) != '\n' && ch != EOF) {
    *p++ = (char)ch;
  }
  *p++ = 0;
  if (ch == EOF) {
      /* Handle EOF or error */
  }
}

After the loop ends, if ch == EOF, the loop has read through to the end of the stream without encountering a newline character, or a read error occurred before the loop encountered a newline character. To conform to FIO34-C. Distinguish between characters read from a file and EOF or WEOF, the error-handling code must verify that an end-of-file or error has occurred by calling feof() or ferror().

Compliant Solution (getchar())

In this compliant solution, characters are no longer copied to buf once index == BUFFERSIZE - 1, leaving room to null-terminate the string. The loop continues to read characters until the end of the line, the end of the file, or an error is encountered. When chars_read > index, the input string has been truncated.

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
 
void func(void) {
  char buf[BUFFERSIZE];
  int ch;
  size_t index = 0;
  size_t chars_read = 0;
 
  while ((ch = getchar()) != '\n' && ch != EOF) {
    if (index < sizeof(buf) - 1) {
      buf[index++] = (char)ch;
    }
    chars_read++;
  }
  buf[index] = '\0';  /* Terminate string */
  if (ch == EOF) {
    /* Handle EOF or error */
  }
  if (chars_read > index) {
    /* Handle truncation */
  }
}

Noncompliant Code Example (fscanf())

In this noncompliant example, the call to fscanf() can result in a write outside the character array buf:

#include <stdio.h>
 
enum { BUF_LENGTH = 1024 };
 
void get_data(void) {
  char buf[BUF_LENGTH];
  if (1 != fscanf(stdin, "%s", buf)) {
    /* Handle error */
  }

  /* Rest of function */
}

Compliant Solution (fscanf())

In this compliant solution, the call to fscanf() is constrained not to overflow buf:

#include <stdio.h>
 
enum { BUF_LENGTH = 1024 };
 
void get_data(void) {
  char buf[BUF_LENGTH];
  if (1 != fscanf(stdin, "%1023s", buf)) {
    /* Handle error */
  }

  /* Rest of function */
}

Noncompliant Code Example (argv)

In a hosted environment, arguments read from the command line are stored in process memory. The function main(), called at program startup, is typically declared as follows when the program accepts command-line arguments:

int main(int argc, char *argv[]) { /* ... */ }

Command-line arguments are passed to main() as pointers to strings in the array members argv[0] through argv[argc - 1]. If the value of argc is greater than 0, the string pointed to by argv[0] is, by convention, the program name. If the value of argc is greater than 1, the strings referenced by argv[1] through argv[argc - 1] are the program arguments.

Vulnerabilities can occur when inadequate space is allocated to copy a command-line argument or other program input. In this noncompliant code example, an attacker can manipulate the contents of argv[0] to cause a buffer overflow:

#include <string.h>
 
int main(int argc, char *argv[]) {
  /* Ensure argv[0] is not null */
  const char *const name = (argc && argv[0]) ? argv[0] : "";
  char prog_name[128];
  strcpy(prog_name, name);
 
  return 0;
}

Compliant Solution (argv)

The strlen() function can be used to determine the length of the strings referenced by argv[0] through argv[argc - 1] so that adequate memory can be dynamically allocated.

#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[]) {
  /* Ensure argv[0] is not null */
  const char *const name = (argc && argv[0]) ? argv[0] : "";
  char *prog_name = (char *)malloc(strlen(name) + 1);
  if (prog_name != NULL) {
    strcpy(prog_name, name);
  } else {
    /* Handle error */
  }
  free(prog_name);
  return 0;
}

Remember to add a byte to the destination string size to accommodate the null-termination character.

Compliant Solution (argv)

The strcpy_s() function provides additional safeguards, including accepting the size of the destination buffer as an additional argument. (See STR07-C. Use the bounds-checking interfaces for string manipulation.)

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[]) {
  /* Ensure argv[0] is not null */
  const char *const name = (argc && argv[0]) ? argv[0] : "";
  char *prog_name;
  size_t prog_size;

  prog_size = strlen(name) + 1;
  prog_name = (char *)malloc(prog_size);

  if (prog_name != NULL) {
    if (strcpy_s(prog_name, prog_size, name)) {
      /* Handle  error */
    }
  } else {
    /* Handle error */
  }
  /* ... */
  free(prog_name);
  return 0;
}

The strcpy_s() function can be used to copy data to or from dynamically allocated memory or a statically allocated array. If insufficient space is available, strcpy_s() returns an error.

Compliant Solution (argv)

If an argument will not be modified or concatenated, there is no reason to make a copy of the string. Not copying a string is the best way to prevent a buffer overflow and is also the most efficient solution. Care must be taken to avoid assuming that argv[0] is non-null.

int main(int argc, char *argv[]) {
  /* Ensure argv[0] is not null */
  const char * const prog_name = (argc && argv[0]) ? argv[0] : "";
  /* ... */
  return 0;
}

Noncompliant Code Example (getenv())

According to the C Standard, 7.22.4.6 [ISO/IEC 9899:2011]

The getenv function searches an environment list, provided by the host environment, for a string that matches the string pointed to by name. The set of environment names and the method for altering the environment list are implementation defined.

Environment variables can be arbitrarily large, and copying them into fixed-length arrays without first determining the size and allocating adequate storage can result in a buffer overflow.

#include <stdlib.h>
#include <string.h>
 
void func(void) {
  char buff[256];
  char *editor = getenv("EDITOR");
  if (editor == NULL) {
    /* EDITOR environment variable not set */
  } else {
    strcpy(buff, editor);
  }
}

Compliant Solution (getenv())

Environmental variables are loaded into process memory when the program is loaded. As a result, the length of these strings can be determined by calling the strlen() function, and the resulting length can be used to allocate adequate dynamic memory:

#include <stdlib.h>
#include <string.h>
 
void func(void) {
  char *buff;
  char *editor = getenv("EDITOR");
  if (editor == NULL) {
    /* EDITOR environment variable not set */
  } else {
    size_t len = strlen(editor) + 1;
    buff = (char *)malloc(len);
    if (buff == NULL) {
      /* Handle error */
    }  
    memcpy(buff, editor, len);
    free(buff);
  }
}

Noncompliant Code Example (sprintf())

In this noncompliant code example, name refers to an external string; it could have originated from user input, the file system, or the network. The program constructs a file name from the string in preparation for opening the file.

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  sprintf(filename, "%s.txt", name);
}

Because the sprintf() function makes no guarantees regarding the length of the generated string, a sufficiently long string in name could generate a buffer overflow.

Compliant Solution (sprintf())

The buffer overflow in the preceding noncompliant example can be prevented by adding a precision to the %s conversion specification. If the precision is specified, no more than that many bytes are written. The precision 123 in this compliant solution ensures that filename can contain the first 123 characters of name, the .txt extension, and the null terminator.

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  sprintf(filename, "%.123s.txt", name);
}

Compliant Solution (snprintf())

A more general solution is to use the snprintf() function:

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  snprintf(filename, sizeof(filename), "%s.txt", name);
}

Risk Assessment

Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code with the permissions of the vulnerable process.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

STR31-C

High

Likely

Medium

P18

L1

Automated Detection

Array access out of bounds, Buffer overflow from incorrect string format specifier, Destination buffer overflow in string manipulation, Invalid use of standard library string routine, Missing null in string array, Pointer access out of bounds, Tainted NULL or non-null-terminated string, Use of dangerous standard function 

Tool

Version

Checker

Description

Astrée
18.10

Supported, but no explicit checker
Axivion Bauhaus Suite6.9.0CertC-STR31

Detects calls to unsafe string function that may cause buffer overflow
Detects potential buffer overruns, including those caused by unsafe usage of fscanf()

CodeSonar
5.0p0

LANG.MEM.BO
LANG.MEM.TO
MISC.MEM.NTERM
BADFUNC.BO.*

Buffer overrun
Type overrun
No space for null terminator
A collection of warning classes that report uses of library functions prone to internal buffer overflows

Compass/ROSE



Can detect violations of the rule. However, it is unable to handle cases involving strcpy_s() or manual string copies such as the one in the first example

Coverity
2017.07

STRING_OVERFLOW

BUFFER_SIZE

OVERRUN

STRING_SIZE

Fully implemented

Fortify SCA

5.0



Klocwork

2018

NNTS.MIGHT
NNTS.MUST
SV.STRBO.BOUND_COPY.OVERFLOW
SV.STRBO.BOUND_COPY.UNTERM
SV.STRBO.BOUND_SPRINTF
SV.STRBO.UNBOUND_COPY
SV.STRBO.UNBOUND_SPRINTF


LDRA tool suite

9.7.1

489 S, 109 D, 66 X, 70 X, 71 X

Partially implemented

Parasoft C/C++test10.4

CERT_C-STR31-a
CERT_C-STR31-b
CERT_C-STR31-c
CERT_C-STR31-d
CERT_C-STR31-e

Avoid accessing arrays out of bounds
Avoid overflow when writing to a buffer
Prevent buffer overflows from tainted data
Avoid buffer write overflow from tainted data
Avoid using unsafe string functions which may cause buffer overflows

Polyspace Bug Finder

R2018a

Array access out of bounds

Buffer overflow from incorrect string format specifier

Destination buffer overflow in string manipulation

Invalid use of standard library string routine

Missing null in string array

Pointer access out of bounds

Tainted NULL or non-null-terminated string

Use of dangerous standard function

Array index outside bounds during array access

String format specifier causes buffer argument of standard library functions to overflow

Function writes to buffer at offset greater than buffer size

Standard library string function called with invalid arguments

String does not terminate with null character

Pointer dereferenced outside its bounds

Argument is from an unsecure source and may be NULL or not NULL-terminated

Dangerous functions cause possible buffer overflow in destination buffer

PRQA QA-C
9.3
2845, 2846, 2847, 2848, 2849, 5009Partially implemented
PRQA QA-C++4.2 0145, 2845, 2846, 2847, 2848, 2849, 2840, 2841, 2842, 2843, 2844, 2930, 2831, 2831, 2832, 2833, 2934 
PVS-Studio

6.23

V518, V645, V727, V755

Splint

3.1.1



Related Vulnerabilities

CVE-2009-1252 results from a violation of this rule. The Network Time Protocol daemon (NTPd), before versions 4.2.4p7 and 4.2.5p74, contained calls to sprintf that allow an attacker to execute arbitrary code by overflowing a character array [xorl 2009].

CVE-2009-0587 results from a violation of this rule. Before version 2.24.5, Evolution Data Server performed unchecked arithmetic operations on the length of a user-input string and used the value to allocate space for a new buffer. An attacker could thereby execute arbitrary code by inputting a long string, resulting in incorrect allocation and buffer overflow [xorl 2009].

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT C Secure Coding StandardSTR03-C. Do not inadvertently truncate a stringPrior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding StandardSTR07-C. Use the bounds-checking interfaces for remediation of existing string manipulation code
MSC24-C. Do not use deprecated or obsolescent functions
MEM00-C. Allocate and free memory in the same module, at the same level of abstraction
FIO34-C. Distinguish between characters read from a file and EOF or WEOF
Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013String Termination [CJM]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013Buffer Boundary Violation (Buffer Overflow) [HCB]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013Unchecked Array Copying [XYW]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013Using a tainted value to write to an object using a formatted input or output function [taintformatio]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013Tainted strings are passed to a string copying function [taintstrcpy]Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-119, Improper Restriction of Operations within the Bounds of a Memory Buffer2017-05-18: CERT: Rule subset of CWE
CWE 2.11CWE-120, Buffer Copy without Checking Size of Input ("Classic Buffer Overflow")2017-05-15: CERT: Exact
CWE 2.11CWE-123, Write-what-where Condition2017-06-12: CERT: Partial overlap
CWE 2.11CWE-125, Out-of-bounds Read2017-05-18: CERT: Partial overlap
CWE 2.11CWE-676, Off-by-one Error2017-05-18: CERT: Partial overlap

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-122 and STR31-C

STR31-C = Union( CWE-122, list) where list =

  • Buffer overflows on strings in the stack or data segment

CWE-125 and STR31-C

Independent( ARR30-C, ARR38-C, EXP39-C, INT30-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

Intersection( STR31-C, CWE-125) =

  • Directly reading beyond the end of a string

STR31-C – CWE-125 =

  • Directly writing beyond the end of a string

CWE-125 – STR31-C =

  • Reading beyond a non-string array
  • Reading beyond a string using library functions

CWE-676 and STR31-C

  • Independent( ENV33-C, CON33-C, STR31-C, EXP33-C, MSC30-C, ERR34-C)
  • STR31-C implies that several C string copy functions, like strcpy() are dangerous.

Intersection( CWE-676, STR31-C) =

  • Buffer Overflow resulting from invocation of the following dangerous functions:
  • gets(), fscanf(), strcpy(), sprintf()

STR31-C – CWE-676 =

  • Buffer overflow that does not involve the dangerous functions listed above.

CWE-676 - STR31-C =

  • Invocation of other dangerous functions

CWE-121 and STR31-C

STR31-C = Union( CWE-121, list) where list =

  • Buffer overflows on strings in the heap or data segment

CWE-123 and STR31-C

Independent(ARR30-C, ARR38-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

Intersection( CWE-123, STR31-C) =

  • Buffer overflow that overwrites a (unrelated) pointer with untrusted data

STR31-C – CWE-123 =

  • Buffer overflow that does not overwrite a (unrelated) pointer

CWE-123 – STR31-C =

  • Arbitrary writes that do not involve buffer overflows

CWE-119 and STR31-C

Independent( ARR30-C, ARR38-C, ARR32-C, INT30-C, INT31-C, EXP39-C, EXP33-C, FIO37-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

CWE-119 = Union( STR31-C, list) where list =

  • Out-of-bounds reads or writes that are not created by string copy operations

CWE-193 and STR31-C

Intersection( CWE-193, STR31-C) = Ø

CWE-193 involves an integer computation error (typically off-by-one), which is often a precursor to (slight) buffer overflow. However the two errors occur in different operations and are thus unrelated.

Bibliography

[Dowd 2006]Chapter 7, "Program Building Blocks" ("Loop Constructs," pp. 327–336)
[Drepper 2006]Section 2.1.1, "Respecting Memory Bounds"
[ISO/IEC 9899:2011]K.3.5.4.1, "The gets_s Function"
[Lai 2006]
[NIST 2006]SAMATE Reference Dataset Test Case ID 000-000-088
[Seacord 2013b]Chapter 2, "Strings"
[xorl 2009]FreeBSD-SA-09:11: NTPd Remote Stack Based Buffer Overflows




19 Comments

  1. This page made me think of a point that I've dwelled on many times before. Consider the compliant solution you have here:

    if (editor) {
      buff = (char *)malloc(strlen(editor)+1);
      strcpy(buff, editor);
    }
    

    In the specific scenario listed, it's fine.. but this code construct can lead to vulnerabilities when some bug exists that causes the string you're copying not to be NUL-terminated correctly. For example, imagine a scenario where due to some previous bug, the 'buf' variable isnt correctly NUL-terminated in the following code (or a NUL byte has been skipped and 'buf' now points to memory out of bounds):

    char *ptr;
    
    ptr = (char *)malloc(strlen(buf));
    if(ptr == NULL)
    &nbsp;&nbsp;&nbsp; return NULL;&nbsp;
    strcpy(ptr, buf);&nbsp;
    

    The problem here is that strlen(buf) can actually change in between the time the string length is taken and when the strcpy() function is performed. For example, if 'buf' was a pointer to the heap, the malloc() call could rearrange the heap so that strlen(buf) changes, and the subsequent strcpy() results in memory corruption. It seems to me like a more defensive and save construct is like this:

    size_t length;
    char *ptr;
    
    length = strlen(buf) + 1;
    ptr = (char *)malloc(length);
    
    if(length == NULL)
    &nbsp;&nbsp;&nbsp; return NULL;
    memcpy(ptr, buf, length);
    

    This second solution will not result in an overflow, even if buf somehow points out of bounds. It would be even better if ptr was explicitly NUL-terminated as well probably. 

    I know it seems like a small point, but I've run into a couple of vulnerabilities of this nature recently, and the use of a fixed length memcpy() or a fixed-length string copy routine vs strcpy() has meant the difference between just a bug vs an exploitable bug. 

  2. I think this rule as it is should be moved to, or have a complimentary rule in, the array section with non-string related examples. For example, the memcpy() example in this rule doesn't have anything to do with NTBS.

    Compared with the other rules in this section, this rule seems have more to do with array mismanagement than the the properties of an NTBS.

  3. The memcpy() example is valid, but is there a good reason to use memcpy() rather than strncpy()?
    AFAICT the only difference is strncpy() zeroes out destination memory after the null terminator.
    (Normmally the null terminator will be the last item in the string, but in a TOCTOU case where
    the src string memory is modified, all bets are off, right?

  4. A better solution is:
    const char *progname=argv[0]; // we're not going to modify either one!
    I have a function in my support library that I use to capture a pointer to the "short" program name (after the path prefix, if any); I extended it to also create argv from interactive input when running in a standalone environment.

    The TOCTU example is bogus, since for multithreaded programming one must in general ensure that access to shared objects are protected by locking critical regions.  And if the src pointer is out of bounds, it is a huge bug to start with, having nothing to do with the current topic.

    The compliant solution for getenv forgot to handle the case where EDITOR is not set.  Proceeding will cause use of an uninitialized "buff".

    1. Added a check for no EDITOR env var to the getenv() examples.

      The TOCTOU example is only partially bogus (smile) You're correct that a vul involving strcpy() in a multithreaded env has bigger problems than strcpy(). The non-compliant example here cause very subtle and difficult bugs in a problematic multi-threaded environment, which are made easier to detect with the compliant solutions.

    2. The TOCTOU example had been bothering me for a while, and I removed it before I even read your comment. Thanks for the moral support, however.

      1. I was going to argue for the memcpy TOCTOU case, but I reread Mark Dowd's comment, which already does an excellent job of arguing the case.

        So I'll simply state that his NCCE & CCE should go somewhere. Perhaps this rule, perhaps another rule, or a new rule of its own, but it should not be forgotten.

  5. is it ok for the rose implementation of this rule to give false positives?

    char buff[] = "sufficient storage";
    strcpy(buff,"abc");
    
    1. In general, yes, but this false positive is easy to spot, as you are copying from a fixed array to another fixed array...it is easy to compare their indices.

      Doesn't that example code also violate STR30-C since the destination array is a string literal?

      1. no, it is an initialized array on the stack

        char * buff = "sufficient storage";
        

        is a string literal in DATA

  6. In paragraph "Noncompliant Code Example (argv) (strcpy())" there is this sentence:

    If the value of argc is greater than zero, the string pointed to by argv[0] is, by convention, the program name.

    All the compliant examples suggest that there is no dependency between the value of argc and the validity of argv[0]. Is this correct, or should error handling occur on argc <= 0?

    1. Good point! Programs should not assume that argc is positive or that argv[0] is non-null. A program with a null name (i.e., an argc of zero) can be executed by invoking

      execv("/path/to/program", (char**)0);
      

      As a data point, virtually every Linux utility in /usr/bin seems to assume that argv[0] is non-null and exits with SIGSEGV when invoked via the call above.

  7. Removed the following:

    The parameters argc and argv and the strings pointed to by the argv array are not modifiable by the program and retain their last-stored values between program startup and program termination. This requires that a copy of these parameters be made before the strings can be modified.

    The standard guarantees the opposite:

    The parameters argc and argv and the strings pointed to by the argv array shall
    be modifiable by the program, and retain their last-stored values between program
    startup and program termination.

  8. The "Compliant Solution (snprintf( ))" example seems to directly violate STR03-C. Do not inadvertently truncate a string. If it's considered intentional truncation, a caution might be appropriate.  (Note that the ".txt" can be truncated away.)

     

    1. I can weasel and suggest that when using snprintf(), we are truncating the input, but not *inadvertently* :)  Keep in mind that STR03-C is a recommendation, not a rule. It is possible to violate CERT recommendations and still have secure code...that is one of the differences between recommendations and rules.  Truncation is acceptable if it is 'intended' (eg you have consciously decided that truncation is acceptable.)

      1. The problem with guidelines like STR03-C. Do not inadvertently truncate a string is that you can't enforce them without knowing the programmer's intent.  This example is too unrealistic to even guess at the intent. In fact, these functions do nothing and would be optimized out into no-ops.

        Other examples in this rule also result in truncation:

          char buf[BUF_LENGTH];
          fscanf(stdin, "%1024s", buf);

        Some of the compliant solutions use dynamic memory allocation, but I think it is a good idea to show some examples with statically allocated buffers.  In these cases, there is nothing much that can be done except to truncate or return an error.  One of these examples could provide a more obvious case for truncation, perhaps writing out the title of an email message as a preview of the message.  These are normally truncated to fit the space available, with maybe the last few characters being overwritten by "..." to show the title has been truncated.

        In other cases like the filename example you might want an error message instead "filename too long".

        1. I did say "if".  It's a little hard to come up with vulnerabilities associated with STR03-C (since simply truncating the input is often equivalent to the villain supplying the truncated input in the first place), but having a program-supplied piece of text like the ".txt" get truncated away seems like one of the ways it can happen.

          The point is that snprintf isn't a panacea; you also must take STR03-C into account and determine whether its truncation will be acceptable.  A comment to that effect on the example seems appropriate.

  9. When strings live on the heap, this rule is a specific instance of MEM35-C. Allocate sufficient memory for an object.

    I'm not sure I believe this. All the code examples and intro text deal exclusively with string copy operations, whereas MEM35-C deals with allocation (on the heap). By that argument, this rule and that rule are mutually exclusive.

    On the other paw, the title is normative and clearly a subset of MEM35-C (when limited to the heap)

    1. IThere is an interesting question here. Consider this code:

      1. char* dest = malloc(size);
      2. strcpy( dest, src);

      If a buffer overflow occurs, is it a violation of STR31-C (buffer overflow on (2)) or MEM35-C (insufficient allocation on (1))?  A static analyzer cannot determine which line was at fault (it could be both).