Buffer Overflow in FreeBSD

Topic: Buffer Overflow in FreeBSD
Versions: All the versions of FreeBSD are broken (4.x, 5.x, 6.0)
Arch: x86
Date: 16/09/2004

All discussion refers to CURRENT-6.0, for other versions some things could change (btw bugged).
Discussion involves a lot of arch x32 dependant mechanisms, so, in some points, could sound a little bit dark.

A buffer overflow has been found in i386/i386/trap.c syscall() function
of FreeBSD official source tree.
In order to rule syscalls mechanism, the 'particular' interrupt 128 (0x80)
is provided in the IDT vector. To serve this interrupt, i386/i386/exception.s int0x80_syscall() function is done and, in the end, it calls syscall().
syscall() is responsible for loading arguments from a syscall and copying
them in a kspace pointer in order to accessing them. The code to do that is the following:


void
syscall(frame)
struct trapframe frame;
{
caddr_t params;
struct sysent *callp;
struct thread *td = curthread;
struct proc *p = td->td_proc;
register_t orig_tf_eflags;
u_int sticks;
int error;
int narg;
int args[8];
u_int code;
...

narg = callp->sy_narg & SYF_ARGMASK; (<- you can see it's
the only one
check)

if (params != NULL && narg != 0)
error = copyin(params, (caddr_t)args,
(u_int)(narg * sizeof(int)));
else
error = 0;

...

and:

> grep SYF_ARGMASK /usr/src/sys/sys/sysent.h
#define SYF_ARGMASK 0x0000FFFF


It's obvious that the amount of selectable memory is beyond the (8 * sizeof(int)) limit of args array, so it would overwrite the saved eip by syscall() (it's invoked through a call) or making an interesting pointer corruption overwriting struct proc *p .

It's exploitable, but the only one way I discovered is to link a new syscall
to the sysent array and to do this you need to be root; I've no time to work on this vulnerability, but i think another way could be found. However it could give serious problems (e.g. kernel crashes).

A good patch could be a dinamyc memory allocation for args, but it's not
a good solution in order to mantain a well performanced system; another one could be a strongest check, but it's not a good solution in order to set a good flexibility.

You would get an attach containing proof of concept code (4.x, 5.x/6.0 versions).


greetings

rookie

P.S: in order to try the code, compile and link module to the kernel, later
do 'make test' and start ./poc

http://seclists.org/lists/bugtraq/2004/Sep/0228.html

谢谢楼主呀…
加3分以资鼓励！

freebsd也来BUG。。。