use-after-free / double-free exploit mitigation

[up201407890 at alunos dot dcc dot fc dot up dot pt]

Hello list,

What are your thoughts on adding a SAFE_FREE() macro to glibc:

#define SAFE_FREE(x) do { if((x) != 0x0) { free(x); (x) = (void *)0x1;  
} } while(0)

After free(x), we set x to an address that will crash when  
dereferenced (use-after-free), and will also crash when it's an  
argument to free(). Note that NULL isn't used, because free(NULL) does  
nothing, which might hide potential double-free bugs.

The pointer passed to free() isn't always an lvalue we can assign to,  
e.g free(func()), but when a var is used it will detect use-after-free  
and double-free bugs by having the program crash instead of allowing  
various heap grooms and further exploitation.


Here's a trivial example,

Crashes when use-after-free:

$ cat test.c
#include <unistd.h>
#include <stdlib.h>

#define SAFE_FREE(x) do { if((x) != 0x0) { free(x); (x)=(void *)0x1; }  
} while(0)

struct unicorn_counter { int num; };

int main() {
    struct unicorn_counter* p_unicorn_counter;
    int* run_calc = malloc(sizeof(int));
    *run_calc = 0;
    SAFE_FREE(run_calc);
    //SAFE_FREE(run_calc);
    p_unicorn_counter = malloc(sizeof(struct unicorn_counter));
    p_unicorn_counter->num = 42;
    if (*run_calc) // use-after-free
       execlp("/usr/bin/id", "id", 0);
}

$ gcc test.c
$ ./a.out
Segmentation fault
$ gdb ./a.out
gdb$ r

...

0x4005ed <main+87>:	mov    eax,DWORD PTR [rax]

Stopped reason: SIGSEGV
0x00000000004005ed in main ()
gdb$ print /x $rax
$1 = 0x1



Crashes when double-free, by uncommenting 2nd SAFE_FREE(run_calc):

$ cat test.c
#include <unistd.h>
#include <stdlib.h>

#define SAFE_FREE(x) do { if((x) != 0x0) { free(x); (x)=(void *)0x1; }  
} while(0)

struct unicorn_counter { int num; };

int main() {
    struct unicorn_counter* p_unicorn_counter;
    int* run_calc = malloc(sizeof(int));
    *run_calc = 0;
    SAFE_FREE(run_calc);
    SAFE_FREE(run_calc); // double-free
    p_unicorn_counter = malloc(sizeof(struct unicorn_counter));
    p_unicorn_counter->num = 42;
    if (*run_calc) execlp("/usr/bin/id", "id", 0);
}

$ gcc test.c
$ gdb ./a.out
gdb$ r

...

0x7ffff7aad614 <__GI___libc_free+20>:	mov    rax,QWORD PTR [rdi-0x8]

Stopped reason: SIGSEGV
__GI___libc_free (mem=0x1) at malloc.c:2929
warning: Source file is more recent than executable.
$ print /x $rdi
$1 = 0x1

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