Android Bug Swatting with Sanitizers

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Posted by Dan Austin, Android Security team

LLVM, the compiler infrastructure used to build Android, contains multiple
components that perform static and dynamic analysis. One set of these components
that have been used extensively when analyzing Android are the sanitizers,
specifically AddressSanitizer, UndefinedBehaviorSanitizer and SanitizerCoverage.
These sanitizers are compiler-based instrumentation components contained in
compiler-rt that can be used in the development and testing process to push out
bugs and make Android better. The sanitizers that are currently available in
Android can discover and diagnose many memory misuse bugs and undefined behavior
and can give code coverage metrics to ensure that your test suite is as complete
as possible.

This blog post details the internals of the current Android
sanitizers—AddressSanitizer, UndefinedBehaviorSanitizer and
SanitizerCoverage—and show how they can be used within the Android build system.

Address Sanitizer

AddressSanitizer
(ASan) is a compiler based instrumentation capability that allows for runtime
detection of many types of memory errors in C/C++ code. In Android, the checks
for the following classes of memory errors have been tested:

  • Out-of-bounds accesses to heap, stack and globals
  • Use-after-free
  • Use-after-return (runtime flag ASAN_OPTIONS=detect_stack_use_after_return=1)
  • Use-after-scope (clang flag -fsanitize-address-use-after-scope)
  • Double-free, invalid free

Android allows for full build instrumentation by ASan, and also allows for ASan
instrumentation at the app level through asanwrapper. Instructions for both
instrumentation techniques can be found on source.android.com.

AddressSanitizer is built upon two high-level concepts. The first is
instrumentation of all memory-related function calls, including alloca, malloc,
and free, with information to track memory allocation, free, and usage
statistics. This instrumentation allows for ASan to detect invalid memory usage
bugs including double-free, use-after scope, return, and free. ASan can also
detect reads and writes that occur out of bounds of defined memory regions. It
does this by padding all allocated memory buffers and variables. If a read or
write to this padding region occurs, ASan catches it and outputs information
useful for diagnosing the memory violation. This padding is known as poisoned
memory in ASan terms. Here is an example of what poisoned memory padding looks
like with stack allocated variables:

Figure 1. Example of ASANified stack variables with an int8_t
array of 8 elements, a uint32_t, and an int8_t array of 16 elements. The memory
layout after compiling with ASAN is on the right, with padding between each
variable. For each stack variable, there are 32 bytes of padding before and
after the variable. If the object size of a variable is not 32 bytes, then an
additional 32 – n bytes of padding are inserted, where n is the object size.

ASan uses shadow memory to keep track of which bytes are normal memory and which
bytes are poisoned memory. Bytes can be marked as completely normal (marked as 0
in shadow memory), completely poisoned (high bit of the corresponding shadow
byte is set), or the first k bytes are unpoisoned (shadow byte value is k). If
shadow memory indicates a byte is poisoned, then ASan crashes the program and
outputs information useful for debugging purposes, including the call stack,
shadow memory map, the type of memory violation, what was read or written, PC
that caused the violation and the memory contents.

AddressSanitizer: heap-buffer-overflow on address 0xe6146cf3 at pc 0xe86eeb3c bp 0xffe67348 sp 0xffe66f14
WRITE of size 39 at 0xe6146cf3 thread T0
    #0 0xe86eeb3b  (/system/lib/libclang_rt.asan-arm-android.so+0x64b3b)
    #1 0xaddc5d27  (/data/simple_test_fuzzer+0x4d27)
    #2 0xaddd08b9  (/data/simple_test_fuzzer+0xf8b9)
    #3 0xaddd0a97  (/data/simple_test_fuzzer+0xfa97)
    #4 0xaddd0fbb  (/data/simple_test_fuzzer+0xffbb)
    #5 0xaddd109f  (/data/simple_test_fuzzer+0x1009f)
    #6 0xaddcbfb9  (/data/simple_test_fuzzer+0xafb9)
    #7 0xaddc9ceb  (/data/simple_test_fuzzer+0x8ceb)
    #8 0xe8655635  (/system/lib/libc.so+0x7a635)
0xe6146cf3 is located 0 bytes to the right of 35-byte region [0xe6146cd0,0xe6146cf3)
allocated by thread T0 here:
    #0 0xe87159df  (/system/lib/libclang_rt.asan-arm-android.so+0x8b9df)
    #1 0xaddc5ca7  (/data/simple_test_fuzzer+0x4ca7)
    #2 0xaddd08b9  (/data/simple_test_fuzzer+0xf8b9)
SUMMARY: AddressSanitizer: heap-buffer-overflow (/system/lib/libclang_rt.asan-arm-android.so+0x64b3b)
Shadow bytes around the buggy address:
  0x1cc28d40: fa fa 00 00 00 00 07 fa fa fa fd fd fd fd fd fd
  0x1cc28d50: fa fa 00 00 00 00 07 fa fa fa fd fd fd fd fd fd
  0x1cc28d60: fa fa 00 00 00 00 00 02 fa fa fd fd fd fd fd fd
  0x1cc28d70: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x1cc28d80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x1cc28d90: fa fa fa fa fa fa fa fa fa fa 00 00 00 00[03]fa
  0x1cc28da0: fa fa 00 00 00 00 07 fa fa fa 00 00 00 00 03 fa
  0x1cc28db0: fa fa fd fd fd fd fd fa fa fa fd fd fd fd fd fa
  0x1cc28dc0: fa fa 00 00 00 00 00 02 fa fa fd fd fd fd fd fd
  0x1cc28dd0: fa fa 00 00 00 00 00 02 fa fa fd fd fd fd fd fd
  0x1cc28de0: fa fa 00 00 00 00 00 02 fa fa fd fd fd fd fd fd
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb

More information on what each part of the report means, and how to make it more
user-friendly can be found on the LLVM website and in
Github.

Sometimes, the bug discovery process can appear to be non-deterministic,
especially when bugs require special setup or more advanced techniques, such as
heap priming or race condition exploitation. Many of these bugs are not
immediately apparent, and could surface thousands of instructions away from the
memory violation that was the actual root cause. As ASan instruments all
memory-related functions and pads data with areas that cannot be accessed
without triggering an ASan-related callback, memory violations are caught the
instant they occur, instead of waiting for a crash-inducing corruption. This is
extremely useful in bug discovery and root cause diagnosis. In addition, ASAN is
an extremely useful tool for fuzzing, and has been used in many
fuzzing
efforts
on Android.

UBSan

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UndefinedBehaviorSanitizer
(UBSan) performs compile-time instrumentation to check for various types of
undefined behavior. Device manufacturers can include it in their test builds by
including LOCAL_SANITIZE:=default-ub in their makefiles or default-ub: true in
the sanitize block of blueprint files. While UBSan can detect many undefined
behaviors, Android’s build system directly supports:

  • bool
  • integer-divide-by-zero
  • return
  • returns-nonnull-attribute
  • shift-exponent
  • unreachable
  • vla-bound

UBSan’s integer overflow checking is also used in Android’s build system. UBSan
also supports unsigned-integer-overflow, which is not technically undefined
behavior, but is included in the sanitizer. These can be enabled in makefiles by
setting LOCAL_SANITIZE to signed-integer-overflow, unsigned-integer-overflow, or
the combination flag integer, which enables signed-integer-overflow,
unsigned-integer-overflow, integer-divide-by-zero, shift-base, and
shift-exponent. These can be enabled in blueprint files by setting
Misc_undefined to the desired flag. These UBSan targets, especially
unsigned-integer-overflow are used extensively in the mediaserver components to
eliminate any latent integer overflow vulnerabilities.

The default implementation on Android is to abort the program when undefined
behavior is encountered. However, starting in October 2016, UBSan on Android has
an optional runtime library that gives more detailed error reporting, including
type of undefined behavior encountered, file and source code line information.

In Android.mk files, this is enabled with:

LOCAL_SANITIZE:=unsigned-integer-overflow signed-integer-overflow
LOCAL_SANITIZE_DIAG:=unsigned-integer-overflow signed-integer-overflow

And in Android.bp files, it is enabled with:

sanitize: {
        misc_undefined: [
            "unsigned-integer-overflow",
            "signed-integer-overflow",
        ],
        diag: {
            misc_undefined: [
                "unsigned-integer-overflow",
                "signed-integer-overflow",
            ],
        },
    },

Here is an example of the information provided by the UBSan runtime library:

external/icu/icu4c/source/common/ucnv.c:1193:23: runtime error: unsigned integer overflow: 4291925010 + 2147483647 cannot be represented in type 'unsigned int'
external/icu/icu4c/source/common/cstring.c:288:16: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'uint32_t' (aka 'unsigned int')
external/harfbuzz_ng/src/hb-private.hh:894:16: runtime error: unsigned integer overflow: 72 - 55296 cannot be represented in type 'unsigned int'
external/harfbuzz_ng/src/hb-set-private.hh:82:24: runtime error: unsigned integer overflow: 32 - 562949953421312 cannot be represented in type 'unsigned long'
system/keymaster/authorization_set.cpp:500:37: runtime error: unsigned integer overflow: 6843601868186924302 * 24 cannot be represented in type 'unsigned long'

SanitizerCoverage

Sanitizer tools have a very simple code coverage tool built in.
SanitizerCoverage allows for code coverage at the call level, basic block level,
or edge level. These can be used as a standalone instrumentation technique or in
conjunction with any of the sanitizers, including AddressSanitizer and
UndefinedBehaviorSanitizer.

To use the new guard-based coverage, set
fsanitize-coverage=trace-pc-guard. This causes the compiler to insert
__sanitizer_cov_trace_pc_guard(&guard_variable) on every edge. Each edge has its
own uint32_t guard_variable. In addition, a module constructor,
__sanitizer_cov_trace_pc_guard_init(uint32_t* start, uint32_t* stop) is also
generated. All the __sanitizer_cov_ functions should be provided by the user.
You can follow the example on Tracing
PCs with guards.

In addition to control flow tracing, SanitizerCoverage allows for data flow
tracing. This is activated with fsanitize-coverage=trace-cmp and is implemented
by instrumenting all switch and comparison instructions with
__sanitizer_cov_trace_* functions. Similar functionality exists for integer
division and GEP instructions, activated with fsanitize-coverage=trace-div and
fsanitize-coverage=trace-gep respectively. This is an experimental interface, is
not thread-safe, and could change at any time, however, it is available and
functional in Android builds.

During a coverage sanitizer session, the coverage information is recorded in two
files, a .sancov file, and a sancov.map file. The first contains all
instrumented points in the program, and the other contains the execution trace
represented as a sequence of indices into the first file. By default, these
files are stored in the current working directory, with one created for each
executable and shared object that ran during the execution.

Conclusion

ASan, UBSan, and SanitizerCoverage are just the beginning of LLVM sanitizer use
in Android. More LLVM Sanitizers are being integrated into the Android build
system. The sanitizers described here can be used as a code health and system
stability mechanism and are even currently being used by Android Security to
find and prevent security bugs!


Android Developers Blog

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