FreeBSD 7.2 VFS/devfs race condition exploit

                                                FreeBSD 7.2 and below (including 6.4) are vulnerable to race condition in VFS
and devfs code, resulting in NULL pointer dereference. In contrast to pipe race
condition, this vulnerability is actually much harder to exploit.

Due to uninitalised value in devfs_open(), following function is called with
fp->f_vnode = 0:

static int
devfs_fp_check(struct file *fp, struct cdev **devp, struct cdevsw **dswp)
{

        *dswp = devvn_refthread(fp->f_vnode, devp);
        if (*devp != fp->f_data) {
                if (*dswp != NULL)
[6]                        dev_relthread(*devp);
                return (ENXIO);
        }
        KASSERT((*devp)->si_refcount > 0,
            ("devfs: un-referenced struct cdev *(%s)", devtoname(*devp)));
        if (*dswp == NULL)
                return (ENXIO);
        curthread->td_fpop = fp;
        return (0);
}


struct cdevsw *
devvn_refthread(struct vnode *vp, struct cdev **devp)
{
        struct cdevsw *csw;
        struct cdev_priv *cdp;

        mtx_assert(&devmtx, MA_NOTOWNED);
        csw = NULL;
        dev_lock();
[1]        *devp = vp->v_rdev;
        if (*devp != NULL) {
[2]                cdp = (*devp)->si_priv;
[3]                if ((cdp->cdp_flags & CDP_SCHED_DTR) == 0) {
[4]                        csw = (*devp)->si_devsw;
                        if (csw != NULL)
[5]                                (*devp)->si_threadcount++;
                }
        }
        dev_unlock();
        return (csw);
}

In [1] vp is dereferenced, resulting in user-controllable *devp pointer (loaded
from *0x1c). If values dereferenced in [2], [3] and [4] are reachable, at [5] we
have memory write at user-controllable address. Unfortunately, the value is
decremented in [6].

In my exploit, I use si_threadcount incrementation to modify kernel code in
devfs_fp_check(). Opcode at 0xc076c64b is "je" (0x74). After incrementation it
changes to 0x75, which is "jne". Such modification results in not calling
dev_relthread() at [6] and eventually leads to function pointer call in
devfs_kqfilter_f().

The following exploit code works only on default 7.2 kernel, due to hardcoded
addresses:

http://www.frasunek.com/devfs.txt

This bug was fixed a week ago and official advisory was issued:

http://security.freebsd.org/advisories/FreeBSD-SA-09:14.devfs.asc

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