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AddressSanitizerAlgorithm

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Mitch Phillips edited this page Oct 4, 2017 · 5 revisions

Short version

The run-time library replaces the malloc and free functions. The memory around malloc-ed regions (red zones) is poisoned. The free-ed memory is placed in quarantine and also poisoned. Every memory access in the program is transformed by the compiler in the following way:

Before:

*address = ...;  // or: ... = *address;

After:

if (IsPoisoned(address)) {
  ReportError(address, kAccessSize, kIsWrite);
}
*address = ...;  // or: ... = *address;

The tricky part is how to implement IsPoisoned very fast and ReportError very compact. Also, instrumenting some of the accesses may be proven redundant.

Memory mapping and Instrumentation

The virtual address space is divided into 2 disjoint classes:

  • Main application memory (Mem): this memory is used by the regular application code.
  • Shadow memory (Shadow): this memory contains the shadow values (or metadata). There is a correspondence between the shadow and the main application memory. Poisoning a byte in the main memory means writing some special value into the corresponding shadow memory.

These 2 classes of memory should be organized in such a way that computing the shadow memory (MemToShadow) is fast.

The instrumentation performed by the compiler:

shadow_address = MemToShadow(address);
if (ShadowIsPoisoned(shadow_address)) {
  ReportError(address, kAccessSize, kIsWrite);
}

Mapping

AddressSanitizer maps 8 bytes of the application memory into 1 byte of the shadow memory.

There are only 9 different values for any aligned 8 bytes of the application memory:

  • All 8 bytes in qword are unpoisoned (i.e. addressable). The shadow value is 0.
  • All 8 bytes in qword are poisoned (i.e. not addressable). The shadow value is negative.
  • First k bytes are unpoisoned, the rest 8-k are poisoned. The shadow value is k. This is guaranteed by the fact that malloc returns 8-byte aligned chunks of memory. The only case where different bytes of an aligned qword have different state is the tail of a malloc-ed region. For example, if we call malloc(13), we will have one full unpoisoned qword and one qword where 5 first bytes are unpoisoned.

The instrumentation looks like this:

byte *shadow_address = MemToShadow(address);
byte shadow_value = *shadow_address;
if (shadow_value) {
  if (SlowPathCheck(shadow_value, address, kAccessSize)) {
    ReportError(address, kAccessSize, kIsWrite);
  }
}

// Check the cases where we access first k bytes of the qword
// and these k bytes are unpoisoned.
bool SlowPathCheck(shadow_value, address, kAccessSize) {
  last_accessed_byte = (address & 7) + kAccessSize - 1;
  return (last_accessed_byte >= shadow_value);
}

MemToShadow(ShadowAddr) falls into the ShadowGap region which is unaddressable. So, if the program tries to directly access a memory location in the shadow region, it will crash.

64-bit

Shadow = (Mem >> 3) + 0x7fff8000;
[0x10007fff8000, 0x7fffffffffff] HighMem
[0x02008fff7000, 0x10007fff7fff] HighShadow
[0x00008fff7000, 0x02008fff6fff] ShadowGap
[0x00007fff8000, 0x00008fff6fff] LowShadow
[0x000000000000, 0x00007fff7fff] LowMem

32 bit

Shadow = (Mem >> 3) + 0x20000000;
[0x40000000, 0xffffffff] HighMem
[0x28000000, 0x3fffffff] HighShadow
[0x24000000, 0x27ffffff] ShadowGap
[0x20000000, 0x23ffffff] LowShadow
[0x00000000, 0x1fffffff] LowMem

Ultra compact shadow

It is possible to use even more compact shadow memory, e.g.

Shadow = (Mem >> 7) | kOffset;

Experiments are in flight.

Report Error

The ReportError could be implemented as a call (this is the default now), but there are some other, slightly more efficient and/or more compact solutions. At some point the default behaviour was:

  • copy the failure address to %rax (%eax).
  • execute ud2 (generates SIGILL)
  • Encode access type and size in a one-byte instruction which follows ud2. Overall these 3 instructions require 5-6 bytes of machine code.

It is possible to use just a single instruction (e.g. ud2), but this will require to have a full disassembler in the run-time library (or some other hacks).

Stack

In order to catch stack buffer overflow, AddressSanitizer instruments the code like this:

Original code:

void foo() {
  char a[8];
  ...
  return;
}

Instrumented code:

void foo() {
  char redzone1[32];  // 32-byte aligned
  char a[8];          // 32-byte aligned
  char redzone2[24];
  char redzone3[32];  // 32-byte aligned
  int  *shadow_base = MemToShadow(redzone1);
  shadow_base[0] = 0xffffffff;  // poison redzone1
  shadow_base[1] = 0xffffff00;  // poison redzone2, unpoison 'a'
  shadow_base[2] = 0xffffffff;  // poison redzone3
  ...
  shadow_base[0] = shadow_base[1] = shadow_base[2] = 0; // unpoison all
  return;
}

Examples of instrumented code (x86_64)

# long load8(long *a) { return *a; }
0000000000000030 <load8>:
  30:	48 89 f8             	mov    %rdi,%rax
  33:	48 c1 e8 03          	shr    $0x3,%rax
  37:	80 b8 00 80 ff 7f 00 	cmpb   $0x0,0x7fff8000(%rax)
  3e:	75 04                	jne    44 <load8+0x14>
  40:	48 8b 07             	mov    (%rdi),%rax   <<<<<< original load
  43:	c3                   	retq   
  44:	52                   	push   %rdx
  45:	e8 00 00 00 00       	callq  __asan_report_load8

# int  load4(int *a)  { return *a; }
0000000000000000 <load4>:
   0:	48 89 f8             	mov    %rdi,%rax
   3:	48 89 fa             	mov    %rdi,%rdx
   6:	48 c1 e8 03          	shr    $0x3,%rax
   a:	83 e2 07             	and    $0x7,%edx
   d:	0f b6 80 00 80 ff 7f 	movzbl 0x7fff8000(%rax),%eax
  14:	83 c2 03             	add    $0x3,%edx
  17:	38 c2                	cmp    %al,%dl
  19:	7d 03                	jge    1e <load4+0x1e>
  1b:	8b 07                	mov    (%rdi),%eax    <<<<<< original load
  1d:	c3                   	retq   
  1e:	84 c0                	test   %al,%al
  20:	74 f9                	je     1b <load4+0x1b>
  22:	50                   	push   %rax
  23:	e8 00 00 00 00       	callq  __asan_report_load4

Unaligned accesses

The current compact mapping will not catch unaligned partially out-of-bound accesses:

int *x = new int[2]; // 8 bytes: [0,7].
int *u = (int*)((char*)x + 6);
*u = 1;  // Access to range [6-9]

A viable solution is described in https://github.com/google/sanitizers/issues/100 but it comes at a performance cost.

Run-time library

Malloc

The run-time library replaces malloc/free and provides error reporting functions like __asan_report_load8.

malloc allocates the requested amount of memory with redzones around it. The shadow values corresponding to the redzones are poisoned and the shadow values for the main memory region are cleared.

free poisons shadow values for the entire region and puts the chunk of memory into a quarantine queue (such that this chunk will not be returned again by malloc during some period of time).

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