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Library Solutions in C/C++

6.2. Library Solutions in C/C++

One partial solution in C/C++ is to use library functions that do not have buffer overflow problems. The first subsection describes the ``standard C library'' solution, which can work but has its disadvantages. The next subsection describes the general security issues of both fixed length and dynamically reallocated approaches to buffers. The following subsections describe various alternative libraries, such as strlcpy and libmib. Note that these don't solve all problems; you still have to code extremely carefully in C/C++ to avoid all buffer overflow situations.

6.2.1. Standard C Library Solution

The ``standard'' solution to prevent buffer overflow in C (which is also used in some C++ programs) is to use the standard C library calls that defend against these problems. This approach depends heavily on the standard library functions strncpy(3) and strncat(3). If you choose this approach, beware: these calls have somewhat surprising semantics and are hard to use correctly. The function strncpy(3) does not NIL-terminate the destination string if the source string length is at least equal to the destination's, so be sure to set the last character of the destination string to NIL after calling strncpy(3). If you're going to reuse the same buffer many times, an efficient approach is to tell strncpy() that the buffer is one character shorter than it actually is and set the last character to NIL once before use. Both strncpy(3) and strncat(3) require that you pass the amount of space left available, a computation that is easy to get wrong (and getting it wrong could permit a buffer overflow attack). Neither provide a simple mechanism to determine if an overflow has occurred. Finally, strncpy(3) has a significant performance penalty compared to the strcpy(3) it supposedly replaces, because strncpy(3) NIL-fills the remainder of the destination. I've gotten emails expressing surprise over this last point, but this is clearly stated in Kernighan and Ritchie second edition [Kernighan 1988, page 249], and this behavior is clearly documented in the man pages for Linux, FreeBSD, and Solaris. This means that just changing from strcpy to strncpy can cause a severe reduction in performance, for no good reason in most cases.

Warning!! The function strncpy(s1, s2, n) can also be used as a way of copying only part of s2, where n is less than strlen(s2). When used this way, strncpy() basically provides no protection against buffer overflow by itself - you have to take separate actions to ensure that n is smaller than the buffer of s1. Also, when used this way, strncpy() does not usually add a trailing NIL after copying n characters. This makes it harder to determine if a program using strncpy() is secure.

You can also use sprintf() while preventing buffer overflows, but you need to be careful when doing so; it's so easy to misapply that it's hard to recommend. The sprintf control string can contain various conversion specifiers (e.g., "%s"), and the control specifiers can have optional field width (e.g., "%10s") and precision (e.g., "%.10s") specifications. These look quite similar (the only difference is a period) but they are very different. The field width only specifies a minimum length and is completely worthless for preventing buffer overflows. In contrast, the precision specification specifies the maximum length that that particular string may have in its output when used as a string conversion specifier - and thus it can be used to protect against buffer overflows. Note that the precision specification only specifies the total maximum length when dealing with a string; it has a different meaning for other conversion operations. If the size is given as a precision of "*", then you can pass the maximum size as a parameter (e.g., the result of a sizeof() operation). This is most easily shown by an example - here's the wrong and right way to use sprintf() to protect against buffer overflows:

 char buf[BUFFER_SIZE];
 sprintf(buf, "%*s",  sizeof(buf)-1, "long-string");  /* WRONG */
 sprintf(buf, "%.*s", sizeof(buf)-1, "long-string");  /* RIGHT */
In theory, sprintf() should be very helpful because you can use it to specify complex formats. Sadly, it's easy to get things wrong with sprintf(). If the format is complex, you need to make sure that the destination is large enough for the largest possible size of the entire format, but the precision field only controls the size of one parameter. The "largest possible" value is often hard to determine when a complicated output is being created. If a program doesn't allocate quite enough space for the longest possible combination, a buffer overflow vulnerability may open up. Also, sprintf() appends a NUL to the destination after the entire operation is complete - this extra character is easy to forget and creates an opportunity for off-by-one errors. So, while this works, it can be painful to use in some circumstances.

Also, a quick note about the code above - note that the sizeof() operation used the size of an array. If the code were changed so that ``buf'' was a pointer to some allocated memory, then all ``sizeof()'' operations would have to be changed (or sizeof would just measure the size of a pointer, which isn't enough space for most values).

The scanf() family is sadly a little murky as well. An obvious question is whether or not the maximum width value can be used in %s to prevent these attacks. There are multiple official specifications for scanf(); some clearly state that the width parameter is the absolutely largest number of characters, while others aren't as clear. The biggest problem is implementations; modern implementations that I know of do support maximum widths, but I cannot say with certainty that all libraries properly implement maximum widths. The safest approach is to do things yourself in such cases. However, few will fault you if you simply use scanf and include the widths in the format strings (but don't forget to count \0, or you'll get the wrong length). If you do use scanf, it's best to include a test in your installation scripts to ensure that the library properly limits length.

6.2.2. Static and Dynamically Allocated Buffers

Functions such as strncpy are useful for dealing with statically allocated buffers. This is a programming approach where a buffer is allocated for the ``longest useful size'' and then it stays a fixed size from then on. The alternative is to dynamically reallocate buffer sizes as you need them. It turns out that both approaches have security implications.

There is a general security problem when using fixed-length buffers: the fact that the buffer is a fixed length may be exploitable. This is a problem with strncpy(3) and strncat(3), snprintf(3), strlcpy(3), strlcat(3), and other such functions. The basic idea is that the attacker sets up a really long string so that, when the string is truncated, the final result will be what the attacker wanted (instead of what the developer intended). Perhaps the string is catenated from several smaller pieces; the attacker might make the first piece as long as the entire buffer, so all later attempts to concatenate strings do nothing. Here are some specific examples:

  • Imagine code that calls gethostbyname(3) and, if successful, immediately copies hostent->h_name to a fixed-size buffer using strncpy or snprintf. Using strncpy or snprintf protects against an overflow of an excessively long fully-qualified domain name (FQDN), so you might think you're done. However, this could result in chopping off the end of the FQDN. This may be very undesirable, depending on what happens next.

  • Imagine code that uses strncpy, strncat, snprintf, etc., to copy the full path of a filesystem object to some buffer. Further imagine that the original value was provided by an untrusted user, and that the copying is part of a process to pass a resulting computation to a function. Sounds safe, right? Now imagine that an attacker pads a path with a large number of '/'s at the beginning. This could result in future operations being performed on the file ``/''. If the program appends values in the belief that the result will be safe, the program may be exploitable. Or, the attacker could devise a long filename near the buffer length, so that attempts to append to the filename would silently fail to occur (or only partially occur in ways that may be exploitable).

When using statically-allocated buffers, you really need to consider the length of the source and destination arguments. Sanity checking the input and the resulting intermediate computation might deal with this, too.

Another alternative is to dynamically reallocate all strings instead of using fixed-size buffers. This general approach is recommended by the GNU programming guidelines, since it permits programs to handle arbitrarily-sized inputs (until they run out of memory). Of course, the major problem with dynamically allocated strings is that you may run out of memory. The memory may even be exhausted at some other point in the program than the portion where you're worried about buffer overflows; any memory allocation can fail. Also, since dynamic reallocation may cause memory to be inefficiently allocated, it is entirely possible to run out of memory even though technically there is enough virtual memory available to the program to continue. In addition, before running out of memory the program will probably use a great deal of virtual memory; this can easily result in ``thrashing'', a situation in which the computer spends all its time just shuttling information between the disk and memory (instead of doing useful work). This can have the effect of a denial of service attack. Some rational limits on input size can help here. In general, the program must be designed to fail safely when memory is exhausted if you use dynamically allocated strings.

6.2.3. strlcpy and strlcat

An alternative, being employed by OpenBSD, is the strlcpy(3) and strlcat(3) functions by Miller and de Raadt [Miller 1999]. This is a minimalist, statically-sized buffer approach that provides C string copying and concatenation with a different (and less error-prone) interface. Source and documentation of these functions are available under a newer BSD-style open source license at ftp://ftp.openbsd.org/pub/OpenBSD/src/lib/libc/string/strlcpy.3.

First, here are their prototypes:

size_t strlcpy (char *dst, const char *src, size_t size);
size_t strlcat (char *dst, const char *src, size_t size);
Both strlcpy and strlcat take the full size of the destination buffer as a parameter (not the maximum number of characters to be copied) and guarantee to NIL-terminate the result (as long as size is larger than 0). Remember that you should include a byte for NIL in the size.

The strlcpy function copies up to size-1 characters from the NUL-terminated string src to dst, NIL-terminating the result. The strlcat function appends the NIL-terminated string src to the end of dst. It will append at most size - strlen(dst) - 1 bytes, NIL-terminating the result.

One minor disadvantage of strlcpy(3) and strlcat(3) is that they are not, by default, installed in most Unix-like systems. In OpenBSD, they are part of <string.h>. This is not that difficult a problem; since they are small functions, you can even include them in your own program's source (at least as an option), and create a small separate package to load them. You can even use autoconf to handle this case automatically. If more programs use these functions, it won't be long before these are standard parts of Linux distributions and other Unix-like systems. Also, these functions have been recently added to the ``glib'' library (I submitted the patch to do this), so using recent versions of glib makes them available. In glib these functions are named g_strlcpy and g_strlcat (not strlcpy or strlcat) to be consistent with the glib library naming conventions.

Also, strlcat(3) has slightly varying semantics when the provided size is 0 or if there are no NIL characters in the destination string dst (inside the given number of characters). In OpenBSD, if the size is 0, then the destination string's length is considered 0. Also, if size is nonzero, but there are no NIL characters in the destination string (in the size number of characters), then the length of the destination is considered equal to the size. These rules make handling strings without embedded NILs consistent. Unfortunately, at least Solaris doesn't (at this time) obey these rules, because they weren't specified in the original documentation. I've talked to Todd Miller, and he and I agree that the OpenBSD semantics are the correct ones (and that Solaris is incorrect). The reasoning is simple: under no condition should strlcat or strlcpy ever examine characters in the destination outside of the range of size; such access might cause core dumps (from accessing out-of-range memory) and even hardware interactions (through memory-mapped I/O). Thus, given:

  a = strlcat ("Y", "123", 0);
The correct answer is 3 (0+3=3), but Solaris will claim the answer is 4 because it incorrectly looks at characters beyond the "size" length in the destination. For now, I suggest avoiding cases where the size is 0 or the destination has no NIL characters. Future versions of glib will hide this difference and always use the OpenBSD semantics.

6.2.4. libmib

One toolset for C that dynamically reallocates strings automatically is the ``libmib allocated string functions'' by Forrest J. Cavalier III, available at http://www.mibsoftware.com/libmib/astring. There are two variations of libmib; ``libmib-open'' appears to be clearly open source under its own X11-like license that permits modification and redistribution, but redistributions must choose a different name, however, the developer states that it ``may not be fully tested.'' To continuously get libmib-mature, you must pay for a subscription. The documentation is not open source, but it is freely available.

6.2.5. C++ std::string class

C++ developers can use the std::string class, which is built into the language. This is a dynamic approach, as the storage grows as necessary. However, it's important to note that if that class's data is turned into a ``char *'' (e.g., by using data() or c_str()), the possibilities of buffer overflow resurface, so you need to be careful when when using such methods. Note that c_str() always returns a NIL-terminated string, but data() may or may not (it's implementation dependent, and most implementations do not include the NIL terminator). Avoid using data(), and if you must use it, don't be dependent on its format.

Many C++ developers use other string libraries as well, such as those that come with other large libraries or even home-grown string libraries. With those libraries, be especially careful - many alternative C++ string classes include routines to automatically convert the class to a ``char *'' type. As a result, they can silently introduce buffer overflow vulnerabilities.

6.2.6. Libsafe

Arash Baratloo, Timothy Tsai, and Navjot Singh (of Lucent Technologies) have developed Libsafe, a wrapper of several library functions known to be vulnerable to stack smashing attacks. This wrapper (which they call a kind of ``middleware'') is a simple dynamically loaded library that contains modified versions of C library functions such as strcpy(3). These modified versions implement the original functionality, but in a manner that ensures that any buffer overflows are contained within the current stack frame. Their initial performance analysis suggests that this library's overhead is very small. Libsafe papers and source code are available at http://www.research.avayalabs.com/project/libsafe. The Libsafe source code is available under the completely open source LGPL license.

Libsafe's approach appears somewhat useful. Libsafe should certainly be considered for inclusion by Linux distributors, and its approach is worth considering by others as well. For example, I know that the Mandrake distribution of Linux (version 7.1) includes it. However, as a software developer, Libsafe is a useful mechanism to support defense-in-depth but it does not really prevent buffer overflows. Here are several reasons why you shouldn't depend just on Libsafe during code development:

  • Libsafe only protects a small set of known functions with obvious buffer overflow issues. At the time of this writing, this list is significantly shorter than the list of functions in this book known to have this problem. It also won't protect against code you write yourself (e.g., in a while loop) that causes buffer overflows.

  • Even if libsafe is installed in a distribution, the way it is installed impacts its use. The documentation recommends setting LD_PRELOAD to cause libsafe's protections to be enabled, but the problem is that users can unset this environment variable... causing the protection to be disabled for programs they execute!

  • Libsafe only protects against buffer overflows of the stack onto the return address; you can still overrun the heap or other variables in that procedure's frame.

  • Unless you can be assured that all deployed platforms will use libsafe (or something like it), you'll have to protect your program as though it wasn't there.

  • LibSafe seems to assume that saved frame pointers are at the beginning of each stack frame. This isn't always true. Compilers (such as gcc) can optimize away things, and in particular the option "-fomit-frame-pointer" removes the information that libsafe seems to need. Thus, libsafe may fail to work for some programs.

The libsafe developers themselves acknowledge that software developers shouldn't just depend on libsafe. In their words:

It is generally accepted that the best solution to buffer overflow attacks is to fix the defective programs. However, fixing defective programs requires knowing that a particular program is defective. The true benefit of using libsafe and other alternative security measures is protection against future attacks on programs that are not yet known to be vulnerable.

6.2.7. Other Libraries

The glib (not glibc) library is a widely-available open source library that provides a number of useful functions for C programmers. GTK+ and GNOME both use glib, for example. As I noted earlier, in glib version 1.3.2, g_strlcpy() and g_strlcat() have been added through a patch which I submitted. This should make it easier to portably use those functions once these later versions of glib become widely available. At this time I do not have an analysis showing definitively that the glib library functions protect against buffer overflows. However, many of the glib functions automatically allocate memory, and those functions automatically fail with no reasonable way to intercept the failure (e.g., to try something else instead). As a result, in many cases most glib functions cannot be used in most secure programs. The GNOME guidelines recommend using functions such as g_strdup_printf(), which is fine as long as it's okay if your program immediately crashes if an out-of-memory condition occurs. However, if you can't accept this, then using such routines isn't appropriate.

    
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