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The readlink() function reads where a link points to. It makes no effort to null-terminate its second argument, buffer. Instead, it just returns the number of characters it has written.

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Noncompliant Code Example

If len is equal to sizeof(buf), the null terminator will be written one is written 1 byte past the end of buf.:

char buf[2561024];
ssize_t len = readlink("/usr/bin/perl", buf, sizeof(buf));
buf[len] = '\0';

A simple (but An incorrect ) solution to this problem is to try to make buf large enough that it can always hold the result:

long symlink_max;
size_t bufsize;
char buf[PATH_MAX+1];
ssize_t *buf;
ssize_t len;

errno = 0;
symlink_max = pathconf("/usr/bin/", _PC_SYMLINK_MAX);
if (symlink_max == -1) {
  if (errno != 0) {
    /* handle error condition */
  }
  bufsize = 10000;
}
else {
  bufsize = symlink_max+1;
}

buf = (char *)malloc(bufsize);
if (buf == NULL) {
  /* handle error condition */
}

len = readlink("/usr/bin/perl", buf, sizeof(bufbufsize));
buf[len] = '\0';

This modification incorrectly assumes that PATH_MAX represents the longest possible path for a file in the filesystem. (PATH_MAX only bounds the longest possible relative path that can be passed to the kernel in a single call.) On most Unix and Linux systems, there is no easily-determined maximum length for a file path, and the symbolic link cannot be longer than the value of SYMLINK_MAX returned by pathconf(). However, the value returned by pathconf() is out of date by the time readlink() is called, so the off-by-one buffer-overflow risk is still present because, between the two calls, the location of /usr/bin/perl can change to a file system with a larger SYMLINK_MAX value. Also, if SYMLINK_MAX is indeterminate (that is, if pathconf() returned -1 without setting errno), the code uses an arbitrary large buffer size (10,000) that it hopes will be sufficient, but there is a small chance that readlink() can return exactly this size.

An additional issue is that readlink() can return -1 if it fails, causing an off-by-one underflow.

Compliant Solution

This example compliant solution ensures there will be is no overflow by only reading in only sizeof(buf)-1 characters. It also properly checks to see if an error has occurred.:

enum { BUFFERSIZE = 2561024 };
char buf[BUFFERSIZE];
ssize_t len;

if ((len = readlink("/usr/bin/perl", buf, sizeof(buf)-1));

if (len != -1) {
    buf[len] = '\0';
}
else {
   /* handle error condition */
}

Risk Assessment

Failing to properly null-terminate the result of readlink() can result in abnormal program termination and buffer-overflow vulnerabilities.

Automated Detection

Related Vulnerabilities

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

References

\[[ilja 06|AA. C References#ilja 06]\]
\[[Open Group 97|AA. C References#Open Group 97]\]
\[[Open Group 04|AA. C References#Open Group 04]\]

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-170 and POS30-C

CWE-170 = Union( POS30-C, list) where list =

• Non-null terminated strings fed to functions other than POSIX readlink()

Bibliography

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Image AddedImage AddedImage AddedPOS02-A. Be careful using errno with multiple threads      50. POSIX (POS)       POS31-C. Do not unlock or destroy another thread's mutex