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2 Hardware-assisted AddressSanitizer Design Documentation
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5 This page is a design document for
6 **hardware-assisted AddressSanitizer** (or **HWASAN**)
7 a tool similar to :doc:`AddressSanitizer`,
8 but based on partial hardware assistance.
14 :doc:`AddressSanitizer`
15 tags every 8 bytes of the application memory with a 1 byte tag (using *shadow memory*),
16 uses *redzones* to find buffer-overflows and
17 *quarantine* to find use-after-free.
18 The redzones, the quarantine, and, to a less extent, the shadow, are the
19 sources of AddressSanitizer's memory overhead.
20 See the `AddressSanitizer paper`_ for details.
22 AArch64 has the `Address Tagging`_ (or top-byte-ignore, TBI), a hardware feature that allows
23 software to use 8 most significant bits of a 64-bit pointer as
24 a tag. HWASAN uses `Address Tagging`_
25 to implement a memory safety tool, similar to :doc:`AddressSanitizer`,
26 but with smaller memory overhead and slightly different (mostly better)
31 * Every heap/stack/global memory object is forcibly aligned by `TG` bytes
32 (`TG` is e.g. 16 or 64). We call `TG` the **tagging granularity**.
33 * For every such object a random `TS`-bit tag `T` is chosen (`TS`, or tag size, is e.g. 4 or 8)
34 * The pointer to the object is tagged with `T`.
35 * The memory for the object is also tagged with `T` (using a `TG=>1` shadow memory)
36 * Every load and store is instrumented to read the memory tag and compare it
37 with the pointer tag, exception is raised on tag mismatch.
39 For a more detailed discussion of this approach see https://arxiv.org/pdf/1802.09517.pdf
46 All memory accesses are prefixed with an inline instruction sequence that
47 verifies the tags. Currently, the following sequence is used:
52 // int foo(int *a) { return *a; }
53 // clang -O2 --target=aarch64-linux -fsanitize=hwaddress -c load.c
55 0: 08 00 00 90 adrp x8, 0 <__hwasan_shadow>
56 4: 08 01 40 f9 ldr x8, [x8] // shadow base (to be resolved by the loader)
57 8: 09 dc 44 d3 ubfx x9, x0, #4, #52 // shadow offset
58 c: 28 69 68 38 ldrb w8, [x9, x8] // load shadow tag
59 10: 09 fc 78 d3 lsr x9, x0, #56 // extract address tag
60 14: 3f 01 08 6b cmp w9, w8 // compare tags
61 18: 61 00 00 54 b.ne 24 // jump on mismatch
62 1c: 00 00 40 b9 ldr w0, [x0] // original load
64 24: 40 20 21 d4 brk #0x902 // trap
66 Alternatively, memory accesses are prefixed with a function call.
71 Tagging the heap memory/pointers is done by `malloc`.
72 This can be based on any malloc that forces all objects to be TG-aligned.
73 `free` tags the memory with a different tag.
78 Stack frames are instrumented by aligning all non-promotable allocas
79 by `TG` and tagging stack memory in function prologue and epilogue.
81 Tags for different allocas in one function are **not** generated
82 independently; doing that in a function with `M` allocas would require
83 maintaining `M` live stack pointers, significantly increasing register
84 pressure. Instead we generate a single base tag value in the prologue,
85 and build the tag for alloca number `M` as `ReTag(BaseTag, M)`, where
86 ReTag can be as simple as exclusive-or with constant `M`.
88 Stack instrumentation is expected to be a major source of overhead,
89 but could be optional.
99 Errors are generated by the `HLT` instruction and are handled by a signal handler.
104 HWASAN uses its own LLVM IR Attribute `sanitize_hwaddress` and a matching
105 C function attribute. An alternative would be to re-use ASAN's attribute
106 `sanitize_address`. The reasons to use a separate attribute are:
108 * Users may need to disable ASAN but not HWASAN, or vise versa,
109 because the tools have different trade-offs and compatibility issues.
110 * LLVM (ideally) does not use flags to decide which pass is being used,
111 ASAN or HWASAN are being applied, based on the function attributes.
113 This does mean that users of HWASAN may need to add the new attribute
114 to the code that already uses the old attribute.
117 Comparison with AddressSanitizer
118 ================================
121 * Is less portable than :doc:`AddressSanitizer`
122 as it relies on hardware `Address Tagging`_ (AArch64).
123 Address Tagging can be emulated with compiler instrumentation,
124 but it will require the instrumentation to remove the tags before
125 any load or store, which is infeasible in any realistic environment
126 that contains non-instrumented code.
127 * May have compatibility problems if the target code uses higher
128 pointer bits for other purposes.
129 * May require changes in the OS kernels (e.g. Linux seems to dislike
130 tagged pointers passed from address space:
131 https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt).
132 * **Does not require redzones to detect buffer overflows**,
133 but the buffer overflow detection is probabilistic, with roughly
134 `(2**TS-1)/(2**TS)` probability of catching a bug.
135 * **Does not require quarantine to detect heap-use-after-free,
136 or stack-use-after-return**.
137 The detection is similarly probabilistic.
139 The memory overhead of HWASAN is expected to be much smaller
140 than that of AddressSanitizer:
141 `1/TG` extra memory for the shadow
142 and some overhead due to `TG`-aligning all objects.
144 Supported architectures
145 =======================
146 HWASAN relies on `Address Tagging`_ which is only available on AArch64.
147 For other 64-bit architectures it is possible to remove the address tags
148 before every load and store by compiler instrumentation, but this variant
149 will have limited deployability since not all of the code is
150 typically instrumented.
152 The HWASAN's approach is not applicable to 32-bit architectures.
157 * `SPARC ADI`_ implements a similar tool mostly in hardware.
158 * `Effective and Efficient Memory Protection Using Dynamic Tainting`_ discusses
159 similar approaches ("lock & key").
160 * `Watchdog`_ discussed a heavier, but still somewhat similar
161 "lock & key" approach.
162 * *TODO: add more "related work" links. Suggestions are welcome.*
165 .. _Watchdog: http://www.cis.upenn.edu/acg/papers/isca12_watchdog.pdf
166 .. _Effective and Efficient Memory Protection Using Dynamic Tainting: https://www.cc.gatech.edu/~orso/papers/clause.doudalis.orso.prvulovic.pdf
167 .. _SPARC ADI: https://lazytyped.blogspot.com/2017/09/getting-started-with-adi.html
168 .. _AddressSanitizer paper: https://www.usenix.org/system/files/conference/atc12/atc12-final39.pdf
169 .. _Address Tagging: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0024a/ch12s05s01.html