Some questions about VM flags

Luca Falavigna
2005-03-23T23:37:19Z

I was playing with mprotect and VM flags when I noticed two curious behaviours.

1)

This C program modifies data segments's flags:

#include <stdio.h>
#include <sys/mman.h>

/* Values shown by /proc/<pid>/maps */
#define START 0x08049000
#define END 0x0804a000

int target;

int main(void)
{
printf("ADDRESS: %p\n", &target);
target = 1;
mprotect((void *)START, END - START, PROT_WRITE);
printf("%d\n", target);
return 0;
}

After the mprotect call, data segment's flags are
08049000-0804a000 -w-p 00000000 03:06 297330 /home/dktrkranz/vmflags_read

Shouldn't printf generate a segfault trying to read a variabile located in a
write-only area?

2)

This C program tries to execute a shellcode:

/* This shellcode calls write and exit. It's harmless ;) */
char shellcode[] =
"\x31\xc0\x31\xdb\x31\xd2\x53\x68\x7a\x0a\x00\x00\x68\x4b\x72"
"\x61\x6e\x68\x44\x6b\x74\x72\x89\xe1\xb2\x0a\xb0\x04\xcd\x80"
"\x31\xc0\x31\xdb\xb0\x01\xcd\x80";

int main(void)
{
void(*sc)(void);
sc = (void *)&shellcode;
sc();
return 0;
}

These are data segment's flags:
08049000-0804a000 rw-p 00000000 03:06 297330 /home/dktrkranz/vmflags_sc

Shellcode lies in this segment. It is executed even if VM_EXEC isn't set. I
think execution shouldn't be permitted if only VM_READ or VM_WRITE flags are
set. Buffer overflows/format string based exploits wouldn't be so popular if we
implemented this feaure. Please let me know your opinion.

Thank you,

Luca