Re: c/r: broken locking when executing map_files

[Vasiliy Kulikov <segoon@openwall.com>]

On Sat, May 05, 2012 at 23:32 +0400, Cyrill Gorcunov wrote:
> On Sat, May 05, 2012 at 10:53:06PM +0400, Cyrill Gorcunov wrote:
> > On Sat, May 05, 2012 at 10:20:51PM +0400, Vasiliy Kulikov wrote:
> > > > -	if (lock_trace(task))
> > > > +	if (!ptrace_may_access(task, PTRACE_MODE_READ))
> > > 
> > > Probably it will be better to change mutex_lock_killable() to
> > > mutex_lock_killable_nested() inside of lock_trace() instead of this change?
> > > It would keep the race-free check.
> > 
> > Yup, if I'm not missing something SINGLE_DEPTH_NESTING should do the trick
> > for us. I'll test and report.
> 
> Hmm, this doesn't work well, the mutex remanins killable so when one does

Does it show circular locking?  It shouldn't block if it uses
mutex_lock+mutex_lock_nested.

>  | [root@neptune ~]# /proc/self/map_files/400000-419000
> 
> it sleeps forever until killed, which is not good I think. Vasiliy, could
> you remind me what exactly is problem if we use unlocked ptrace_may_access
> here?

There is a race between ptrace_may_access() and dname_to_vma_addr().
The target task may do exec() between these calls and
dname_to_vma_addr() will be called against a privileged task.  This may
lead to a leak of task maps.

-- 
Vasily Kulikov
http://www.openwall.com - bringing security into open computing environments