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The trouble with snprintf

fweimer@redhat.com published on 2014-03-12T13:30:40+00:00, last updated 2014-03-12T13:30:40+00:00

At least historically, misuse of functions like strcpy, strcat, and sprintf was a common source of buffer overflow vulnerabilities. Therefore, in 1997, the Single UNIX Specification, Version 2, included a new interface for string construction that provided an explicit length of the output string: snprintf. This function can be used for string construction with explicit length checking.

Originally, it could be used in the following way:

    /* buff is a pointer to a buffer of blen characters. */
    /* Note well: This example is now incorrect. */
    char *cp = buff;
    if ((n = snprintf(cp, blen, "AF=%d ", sau->soa.sa_family)) < 0) {
        Warn1("sockaddr_info(): buffer too short ("F_Zu")", blen);
        *buff = '';
        return buff;
    }
    cp += n,  blen -= n;

This example is based on socat, but this coding pattern is still fairly common. The socat case is likely harmless as far as potential security impact is concerned.

The code in the above example avoids writing too much data to the buff pointer because early snprintf implementations returned -1 if the output string was truncated, based on this requirement from the Single UNIX Specification, Version 2:

Upon successful completion, these functions return the number of bytes transmitted excluding the terminating null in the case of sprintf() or snprintf() or a negative value if an output error was encountered.

However, the example code is insecure when compiled on current systems.

The specification quoted above is still ambiguous with regard to truncation, something that would be addressed during the standardization of the next version of C, ISO C99. As a result of that, in 2002, version 3 of the Single UNIX Specification was published, aligning the snprintf behavior with ISO C99:

Upon successful completion, the snprintf() function shall return the number of bytes that would be written to s had n been sufficiently large excluding the terminating null byte.

There is a remaining discrepancy between ISO C99 and POSIX regarding the EOVERFLOW return value, but we will ignore that. As far as the history can be retraced now, the GNU C Library adopted the ISO C99 behavior some time in 1998.

After this specification change, truncated output does not result in an error return value any more. Even worse, the result value exceeds the passed buffer length, making the pointer and length adjustment in the example invalid:

    cp += n,  blen -= n;

If the cp pointer and the blen argument are used in subsequent snprintf calls (which is often the case when the result from snprintf is used in pointer arithmetic), the buffer overflow vulnerability that snprintf was supposed to deal with resurfaces: cp points outside of the original buffer, and blen wraps around (after the conversion in size_t), resulting in a value that does not stop snprintf from writing to the invalid pointer.

Covering this error condition is somewhat difficult:

    char *cp = buff;
    n = snprintf(cp, blen, "AF=%d ", sau->soa.sa_family)
    if (n < 0) {
        Warn1("sockaddr_info(): snprintf failed: %s", strerror(errno));
        *buff = '';
        return buff;
    } else if (n >= blen) {
        Warn1("sockaddr_info(): buffer too short ("F_Zu")", blen);
        *buff = '';
        return buff;
    }
    cp += n,  blen -= n;

As a more convenient substitute, it is possible to ignore the return value from snprintf altogether, and acquire the number of written characters using strlen:

    char *cp = buff;
    assert(blen >= 1);
    *buff = 0;
    snprintf(cp, blen, "AF=%d ", sau->soa.sa_family)
    blen -= strlen(cp);
    cp += strlen(cp);

This code assumes that the snprintf implementation does not write an unterminated string to the destination buffer on error, which is quite reasonable as far as such assumptions go.  The value of strlen(cp) will always be less than the value of blen, so a subsequent snprintf will have room to write the null terminator.

Enhancing -D_FORTIFY_SOURCE=2 to cover the original example code reliably is difficult because GCC cannot track the size information through the pointer arithmetic following the snprintf call, so it is not available to subsequent snprintf calls. Another option would be to have snprintf abort in fortify mode when the buffer length passed in is INT_MAX or larger. Adding logic to GCC to deal with this snprintf oddity specifically is a bit dubious, considering that this only deals with misuse of a single library function.

Curiously, snprintf is not the only function that suffered from an interface change as the result of standardization. Another example is strerror_r, the thread-safe variant of strerror. Even today, it exists in two variants in the GNU C library, one that returns a pointer value (used with -D_GNU_SOURCE) and one that returns an int (the standardized version).

One can only hope that with increased openness of standardization processes and more participation from the free software community in the creation of mostly proprietary standard documents, future recurrences of this kind of problem can be avoided, for example by standardizing interfaces with conflicting implementations under completely new names.

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