This post was originally published at FORTIFY in Android

Posted by George Burgess, Software Engineer

FORTIFY is an important security feature that’s been available in Android since
mid-2012. After migrating from GCC to clang as the default C/C++ compiler early last
year, we invested a lot of time and effort to ensure that FORTIFY on clang is of
comparable quality. To accomplish this, we redesigned how some key FORTIFY
features worked, which we’ll discuss below.

Before we get into some of the details of our new FORTIFY, let’s go through a
brief overview of what FORTIFY does, and how it’s used.

What is FORTIFY?

FORTIFY is a set of extensions to the C standard library that tries to catch the
incorrect use of standard functions, such as memset, sprintf, open, and others.
It has three primary features:

  • If FORTIFY detects a bad call to a standard library function at
    compile-time, it won’t allow your code to compile until the bug is fixed.
  • If FORTIFY doesn’t have enough information, or if the code is definitely
    safe, FORTIFY compiles away into nothing. This means that FORTIFY has 0 runtime
    overhead when used in a context where it can’t find a bug.
  • Otherwise, FORTIFY adds checks to dynamically determine if the questionable
    code is buggy. If it detects bugs, FORTIFY will print out some debugging
    information and abort the program.

Consider the following example, which is a bug that FORTIFY caught in real-world
code:

struct Foo 
    int val;
    struct Foo *next;
;
void initFoo(struct Foo *f) 
    memset(&f, 0, sizeof(struct Foo));

FORTIFY caught that we erroneously passed &f as the first argument to memset,
instead of f. Ordinarily, this kind of bug can be difficult to track down: it
manifests as potentially writing 8 bytes extra of 0s into a random part of your
stack, and not actually doing anything to *f. So, depending on your compiler
optimization settings, how initFoo is used, and your project’s testing
standards, this could slip by unnoticed for quite a while. With FORTIFY, you get
a compile-time error that looks like:

/path/to/file.c: call to unavailable function 'memset': memset called with size bigger than buffer
    memset(&f, 0, sizeof(struct Foo));
    ^~~~~~

For an example of how run-time checks work, consider the following function:

// 2147483648 == pow(2, 31). Use sizeof so we get the nul terminator,
// as well.
#define MAX_INT_STR_SIZE sizeof("2147483648")
struct IntAsStr 
    char asStr[MAX_INT_STR_SIZE];
    int num;
;
void initAsStr(struct IntAsStr *ias) 
    sprintf(ias->asStr, "%d", ias->num);

This code works fine for all positive numbers. However, when you pass in an
IntAsStr with num <= -1000000, the sprintf will write MAX_INT_STR_SIZE+1 bytes
to ias->asStr. Without FORTIFY, this off-by-one error (that ends up clearing one
of the bytes in num) may go silently unnoticed. With it, the program prints out
a stack trace, a memory map, and will abort with a core dump.

FORTIFY also performs a handful of other checks, such as ensuring calls to open have the proper
arguments, but it’s primarily used for catching memory-related errors like the
ones mentioned above.

However, FORTIFY can’t catch every memory-related bug that exists. For
example, consider the following code:

__attribute__((noinline)) // Tell the compiler to never inline this function.
inline void intToStr(int i, char *asStr)  sprintf(asStr, “%d”, num); 


char *intToDupedStr(int i) 
    const int MAX_INT_STR_SIZE = sizeof(“2147483648”);
    char buf[MAX_INT_STR_SIZE];
    intToStr(i, buf);
    return strdup(buf);

Because FORTIFY determines the size of a buffer based on the buffer’s type
and—if visible—its allocation site, it can’t catch this bug. In this case,
FORTIFY gives up because:

  • the pointer is not a type with a pointee size we can determine with
    confidence because char * can point to a variable amount of bytes
  • FORTIFY can’t see where the pointer was allocated, because
    asStr could point to anything.

If you’re wondering why we have a noinline attribute there, it’s because FORTIFY
may be able to catch this bug if intToStr gets inlined into intToDupedStr. This
is because it would let the compiler see that asStr points to the same memory as
buf, which is a region of sizeof(buf) bytes of memory.

How FORTIFY works

FORTIFY works by intercepting all direct calls to standard library functions at
compile-time, and redirecting those calls to special FORTIFY’ed versions of said
library functions. Each library function is composed of parts that emit run-time
diagnostics, and—if applicable—parts that emit compile-time diagnostics. Here is
a simplified example of the run-time parts of a FORTIFY’ed memset (taken from
string.h). An actual FORTIFY implementation may include a few extra
optimizations or checks.

_FORTIFY_FUNCTION
inline void *memset(void *dest, int ch, size_t count) 
    size_t dest_size = __builtin_object_size(dest);
    if (dest_size == (size_t)-1)
        return __memset_real(dest, ch, count);
    return __memset_chk(dest, ch, count, dest_size);

In this example:

  • _FORTIFY_FUNCTION expands to a handful of compiler-specific attributes to
    make all direct calls to memset call this special wrapper.
  • __memset_real is used to bypass FORTIFY to call the “regular” memset
    function.
  • __memset_chk is the special FORTIFY’ed memset. If count > dest_size,
    __memset_chk aborts the program. Otherwise, it simply calls through to
    __memset_real.
  • __builtin_object_size is where the magic happens: it’s a lot like size
    sizeof, but instead of telling you the size of a type, it tries to figure out
    how many bytes exist at the given pointer during compilation. If it fails, it
    hands back (size_t)-1.

The __builtin_object_size might seem sketchy. After all, how can the compiler
figure out how many bytes exist at an unknown pointer? Well… It can’t. 🙂 This
is why _FORTIFY_FUNCTION requires inlining for all of these functions: inlining
the memset call might make an allocation that the pointer points to (e.g. a
local variable, result of calling malloc, …) visible. If it does, we can often
determine an accurate result for __builtin_object_size.

The compile-time diagnostic bits are heavily centered around
__builtin_object_size, as well. Essentially, if your compiler has a way to emit
diagnostics if an expression can be proven to be true, then you can add that to
the wrapper. This is possible on both GCC and clang with compiler-specific
attributes, so adding diagnostics is as simple as tacking on the correct
attributes.

Why not Sanitize?

If you’re familiar with C/C++ memory checking tools, you may be wondering why
FORTIFY is useful when things like clang’s
AddressSanitizer
exist. The sanitizers are excellent for catching and
tracking down memory-related errors, and can catch many issues that FORTIFY
can’t, but we recommend FORTIFY for two reasons:

  • In addition to checking your code for bugs while it’s running, FORTIFY can
    emit compile-time errors for code that’s obviously incorrect, whereas the
    sanitizers only abort your program when a problem occurs. Since it’s generally
    accepted that catching issues as early as possible is good, we’d like to give
    compile-time errors when we can.
  • FORTIFY is lightweight enough to enable in production. Enabling it on parts
    of our own code showed a maximum CPU performance degradation of ~1.5% (average
    0.1%), virtually no memory overhead, and a very small increase in binary size.
    On the other hand, sanitizers can slow code down by well over 2x, and often eat
    up a lot of memory and storage space.

Because of this, we enable FORTIFY in production builds of Android to mitigate
the amount of damage that some bugs can cause. In particular, FORTIFY can turn
potential remote code execution bugs into bugs that simply abort the broken
application. Again, sanitizers are capable of detecting more bugs than FORTIFY,
so we absolutely encourage their use in development/debugging builds. But the
cost of running them for binaries shipped to users is simply way too high to
leave them enabled for production builds.

FORTIFY redesign

FORTIFY’s initial implementation used a handful of tricks from the world of C89,
with a few GCC-specific attributes and language extensions sprinkled in. Because
Clang cannot emulate how GCC works to fully support the original FORTIFY
implementation, we redesigned large parts of it to make it as effective as
possible on clang. In particular, our clang-style FORTIFY implementation makes
use of clang-specific attributes and language extensions, as well as some
function overloading (clang will happily apply C++ overloading rules to your C
functions if you use its overloadable attribute).

We tested hundreds of millions of lines of code with this new FORTIFY, including
all of Android, all of Chrome OS (which needed its own reimplementation of
FORTIFY), our internal codebase, and many popular open source projects.

This testing revealed that our approach broke existing code in a variety of
exciting ways, like:

template <typename OpenFunc>
bool writeOutputFile(OpenFunc &&openFile, const char *data, size_t len) {}

bool writeOutputFile(const char *data, int len) 
    // Error: Can’t deduce type for the newly-overloaded `open` function.
    return writeOutputFile(&::open, data, len);

and

We recently pushed it to AOSP, and starting in Android O, the Android platform
will be protected by clang FORTIFY. We’re still putting some finishing touches
on the NDK, so developers should expect to see our upgraded FORTIFY
implementation there in the near future. In addition, as we alluded to above,
Chrome OS also has a similar FORTIFY implementation now, and we hope to work
with the open-source community in the coming months to get a similar
implementation* into glibc, the
GNU C library.


* For those who are interested, this will look very different than the Chrome OS
patch. Clang recently gained an attribute called diagnose_if,
which ends up allowing for a much cleaner FORTIFY implementation than
our original approach for glibc, and produces far prettier errors/warnings than
we currently can. We expect to have a similar diagnose_if-powered implementation
in a later version of Android.

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This post was originally published at FORTIFY in Android