Re: [PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Sun, Oct 4, 2009 at 10:48 PM, KOSAKI Motohiro
> <kosaki.motohiro@jp.fujitsu.com> wrote:
> 
> > +                       } else {
> > +                               len = mm->env_end - mm->env_start;
> > +                               if (len > PAGE_SIZE - res)
> > +                                       len = PAGE_SIZE - res;
> > +                               res += access_process_vm(task, mm->env_start,
> > +                                                        buffer+res, len, 0);
> > +                               res = strnlen(buffer, res);
> > +                       }
> 
> 
> This bug was in the original code, but since you're touching it
> anyway, it should be fixed now; if this access_process_vm fails
> (perhaps due to the target unmapping the page in question in between
> the two calls), bad things might happen if (error code) + res < 0, as
> then strnlen will get a huge value in its length (possibly leading to
> OOPS etc). It should be changed to check for an error return here and
> fail out properly if there is an error in this second check.

AFAIK, access_process_vm() never return negative value.

===================================================================
int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write)
{
        mm = get_task_mm(tsk);
        if (!mm)
                return 0;

        down_read(&mm->mmap_sem);
        /* ignore errors, just check how much was successfully transferred */
        while (len) {
(snip)

        }
        up_read(&mm->mmap_sem);
        mmput(mm);

        return buf - old_buf;
}