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New define in mem.c DEBUG_USE_AFTER_FREE_ON_WINDOWS which helps catch use-after-free issues in a debugger on Windows #737

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New define in mem.c DEBUG_USE_AFTER_FREE_ON_WINDOWS which helps catch use-after-free issues in a debugger on Windows #737

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76 changes: 76 additions & 0 deletions src/mem.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,13 @@

#include <stdlib.h>

// Enable this to catch use-after-free issues when debugging on Windows.
// #define DEBUG_USE_AFTER_FREE_ON_WINDOWS

#if !defined(DEBUG_USE_AFTER_FREE_ON_WINDOWS)

// The standard implementation, using malloc() and free().

void * avifAlloc(size_t size)
{
void * out = malloc(size);
Expand All @@ -18,3 +25,72 @@ void avifFree(void * p)
{
free(p);
}

#else

// This implementation rounds up all memory allocations to the nearest 4k (page size), allocates
// using VirtualAlloc(), and then records the allocation in a linked list. When avifFree is called,
// instead of freeing the memory, it simply revokes all read and write access to that region
// forever. If any use-after-free issues are discovered (via fuzzing or the like), enabling this
// should immediately catch it in a debugger right when it happens.

#include <stdio.h>
#include <windows.h>

typedef struct Allocation
{
void * ptr;
size_t originalSize;
size_t size;
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Optional: If I understand the MSDN pages for VirtualAlloc() and VirtualProtect() correctly, we can just pass the original size to these two functions and they will round the size up to the next page boundary for us.

struct Allocation * next;
} Allocation;

static Allocation * allocations = NULL;

void * avifAlloc(size_t size)
{
size_t originalSize = size;
size = (size + 4095) & ~(4095);

void * out = VirtualAlloc(NULL, size, MEM_COMMIT, PAGE_READWRITE);
if (out == NULL) {
abort();
}

Allocation * a = (Allocation *)malloc(sizeof(Allocation));
a->ptr = out;
a->originalSize = originalSize;
a->size = size;
a->next = allocations;
allocations = a;

// printf("Alloc %p %zu (%zu)\n", a->ptr, a->originalSize, a->size);
return out;
}

void avifFree(void * p)
{
if (!p) {
return;
}

Allocation * a = allocations;
for (; a != NULL; a = a->next) {
if (a->ptr == p) {
break;
}
}

if (!a) {
abort();
}

DWORD old;
if (!VirtualProtect(a->ptr, a->size, PAGE_NOACCESS, &old)) {
abort();
}

// printf("Free %p %zu (%zu)\n", a->ptr, a->originalSize, a->size);
}

#endif