App Development



Software for developing Android apps

Android Developers Blog  May 27, 2017 – 09:26 am

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

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

Source: android-developers.googleblog.com

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