https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82179
--- Comment #1 from Federico Bento <up201407890 at fc dot up.pt> ---
Hello,
This a request for an exploit mitigation to be added to the compiler, possibly
when making use of FORTIFY_SOURCE.
Something like the below, but at the compiler level:
#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.
This will detect use-after-free and double-free bugs by having the program
crash instead of allowing various heap grooms and further exploitation.
After discussion with Martin Sebor and Florian Weimer in the libc-alpha list,
it was pointed out to me to post the request here for further interest.
https://sourceware.org/ml/libc-alpha/2017-09/msg00238.html
https://sourceware.org/ml/libc-alpha/2017-09/msg00423.html
Below are some examples of how it works,
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
