SecuriTeam - Kfence, Kernel Protection against Basic Exploitation Techniques

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Kfence provides kernel protection against basic exploitation techniques, including stack and heap overflows and format string exploit, by patching /dev/kmem and redirecting system_call to test if the EIP of the caller is in the wrong memory region.

Tool code:
/*
* kfence 1.2
* added .bss exec protection
* modified extraction of struct distances
* you need to have a valid System.map on your box
* tested on 2.2.16 2.4.7 2.4.18 2.4.19 2.4.10
* Coded by ins1der
* 2003
* trixterjack@yahoo.com
*/

#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <asm/unistd.h>
#define ulong unsigned long
#define systemmap "/boot/System.map"
struct _idt
{
  unsigned short offset_low,segment_sel;
  unsigned char reserved,flags;
  unsigned short offset_high;
};

char shellc[]=
                                /*start:*/
"\x83\xf8\x21" /*cmp $0x21,%eax*/
"\x74\x32" /*je exitme*/
"\x55" /*push %ebp*/
"\x89\xe5" /*mov %esp,%ebp*/
"\x60" /*pusha*/
"\x8b\x55\x04" /*mov 0x4(%ebp),%edx*/
"\x81\xfa\xfe\xff\xff\xbf" /*cmp $0xbffffffe,%edx*/
"\x77\x08" /*ja test1*/
"\x81\xfa\x00\x00\x00\xbf" /*cmp $0xbf000000,%edx*/
"\x77\x1b" /*ja exitme*/
/*test1:*/
"\xb8\x00\xe0\xff\xff" /*mov $0xffffe000,%eax*/
"\x21\xe0" /*and %esp,%eax*/
"\x8b\x80\x00\x00\x00\x00" /*mov 0x0(%eax),%eax*/
"\x3b\x50\x00" /*cmp 0x0(%eax),%edx*/
"\x76\x05" /*jbe endme*/
"\x3b\x50\x00" /*cmp 0x0(%eax),%edx*/
"\x72\x04" /*jb exitme*/
/*endme:*/
"\x61" /*popa*/
"\x5d" /*pop %ebp*/
"\xeb\x06" /*jmp realend*/
/*exitme:*/
"\x31\xc0" /*xor %eax,%eax*/
"\xfe\xc0" /*inc %al*/
"\xcd\x80" /*int $0x80*/
/*realend:*/
;
int verbose=0;
/* read or write to /dev/kmem */

int kmemrw(void *rwbuf, ulong offset, size_t size, int type)
{
  int f;
  int ret;
  f=open("/dev/kmem",O_RDWR);
  if (f<0)
    {
fprintf(stderr,"unable to open /dev/kmem in rw mode\n");
_exit(0);
    }
  lseek(f,offset,SEEK_SET);
  if (type==0)
    ret=read(f,rwbuf,size);
  else
    ret=write(f,rwbuf,size);
  close(f);
  return ret;
}

void usage(char *argv)
{
  fprintf(stderr,"***\nkfence \nins1der 2003
(trixterjack@yahoo.com)\n***\n");
  fprintf(stderr,"Usage : %s command [v]\n",argv);
  fprintf(stderr,"Commands:\n");
  fprintf(stderr," r remove the kernel patch\n");
  fprintf(stderr," i install kfence \n");
  _exit(0);
}

int main(int argc,char *argv[])
{

  unsigned char idtr[6],shell[100],start_data;
  struct _idt idt;
  ulong olduname,sct,len,system_call,readbytes,mm_dist=0;
  ulong load_aout;
  char *p;

  if ((argc<2)||(argv[1][0]!='r'&&argv[1][0]!='i')) usage(argv[0]);
if (argc==3)
{
if (argv[2][0]!='v') usage(argv[0]);
else verbose=1;
}
if (argv[1][0]=='i')
printf("Starting kfence 1.2\n");
else
printf("Removing kfence 1.2\n");

{
FILE *f;
char buf[256];
f=fopen(systemmap,"r");
if (f==NULL)
  {
   printf("System.map path not good maybe?\n");
   _exit(0);
  }

do
{
fgets(buf,sizeof(buf),f);
if (memmem(buf,256,"load_aout_interp",16)) break;
}
while (!feof(f));

if (feof(f))
{
printf("load_aout_interp not found\n");
fclose(f);
_exit(0);
}

sscanf(buf,"%x",(int*)&load_aout);

fclose(f);
}

  /*get the interrupt descriptor table*/

  asm("sidt %0" : "=m" (idtr));

  /*get the address of system_call*/

  kmemrw(&idt,*((ulong*)&idtr[2])+8*0x80,sizeof(idt),0);
  system_call=(idt.offset_high<<16)|idt.offset_low;

if (verbose)
  printf("# system_call at 0x%x\n",(int)system_call);

  /*read the first 4 bytes of system_call*/

  kmemrw(&readbytes,system_call,4,0);


  if (argv[1][0]=='r')

    {

      if (readbytes==0x1e06fc50)

/*if the first bytes of system_call are normal kfence is not
installed*/

printf("kfence is not up. nothing to remove.\n");

      else

{

/*remove kfence by writing the usual bytes back in system_call*/

kmemrw("\x50\xfc\x06\x1e\x50\x55",system_call,6,1);
printf("Done.\n");

}

      return 0;

    }


  if (readbytes!=0x1e06fc50)
    {

      fprintf(stderr,"Something is wrong with system_call\n");
      fprintf(stderr,"kfence probably installed\n");
      fprintf(stderr,"Exiting.\n");
      _exit(0);

    }
if (verbose)
printf("# load_aout_interp at 0x%x\n",(int)load_aout);


  /*get sys_call_table*/

  kmemrw(shell,system_call,100,0);
  p=(char*)memmem(shell,100,"\xff\x14\x85",3);
  if (!p)
    {
      fprintf(stderr,"sys_call_table not found!\n");
      return 0;
    }
  p+=3;
  sct=*(ulong*)p;
if (verbose)
  printf("# sys_call_table 0x%x\n",(int)sct);

  /*get the address of sys_olduname*/

  kmemrw(&olduname,sct+4*__NR_oldolduname,4,0);
if (verbose)
  printf("# olduname at 0x%x\n",(int)olduname);

  /*get the distances from load_aout_interp bling bling! :\*/



  p=(char*)kmemrw(shell,load_aout,70,0);
  p=(char*)memmem(shell,70,"\x00\xe0\xff\xff",4);
  if (!p)
    {
    fprintf(stderr,"couldn't get needed structures!\n");
    return 0;
    }

  p--;
  {
  unsigned char i,j,b,c;
  i=*p;
  i-=0xb8;
for (;;)
{
  p=(char*)memchr(p,'\x8b',70-((ulong)p-(ulong)shell));
    if (!p)
{
fprintf(stderr,"couldn't get needed structures!\n");
return 0;
}
  p++;
  b=*p;
  b-=0x40;

  if (b<=0x3f)

if (b%0x8==i)
{
p++;
mm_dist=*p;
for (;;)
{
   j=b/0x8;
   p=(char*)memchr(p,'\x89',70-((ulong)p-(ulong)shell));
       if (!p)
{
fprintf(stderr,"couldn't get needed structures!\n");
return 0;
}

p++;
   c=*p;
   c-=0x40;
   if (c%0x8==j)
   {
   p++;
start_data=*p+4;
   break;
   }
}
  break;
}


if((b>=0x40)&&(b<=0x80))
  {
b-=0x40;
   if (b%0x8==i)
{
p++;
   mm_dist=*(int*)p;
p+=6;
        for (;;)
{
   j=b/0x8;
     p=(char*)memchr(p,'\x89',70-((ulong)p-(ulong)shell));
         if (!p)
                    {
                           fprintf(stderr,"couldn't get needed structures!\n");
                    return 0;
                    }
     p++;
       c=*p;
        c-=0x40;
   if (c%0x8==j)
     {
p++;
start_data=*p+4;
break;
     }
}
break;
}

  }

}

  }

  if (verbose)
{
  printf("# mm distance in task_struct = 0x%x\n",(int)mm_dist);
  printf("# start_data distance in mm_struct = 0x%x\n",start_data);
  printf("##Sleeping for 2 seconds.\n");
}
  sleep(1);
  printf(".");
  fflush(stdout);
  sleep(1);
  printf(".\n");

  /*prepare our code*/

  len=sizeof(shellc)-1;
  shellc[2]=__NR_oldolduname;
  *(int*)(shellc+37)=mm_dist;
  shellc[43]=start_data;
  shellc[48]=start_data+0xc;
  memcpy(shell,shellc,len);
  kmemrw(shell+len,system_call,6,0);
  *(char*)(shell+len+6)=0x68;
  *(int*)(shell+len+7)=(int)(system_call+6);
  *(char*)(shell+len+11)=0xc3;

  /*write the code in sys_olduname*/

  kmemrw(shell,olduname,len+12,1);
  shell[0]=0x68;
  *(int*)(shell+1)=olduname;
  shell[5]=0xc3;

  /*write a push ret at the begining system_call to jump to
sys_olduname*/

  kmemrw(shell,system_call,6,1);

  printf("Done.\n");

  return 1;
}

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