Creating tasks on restart: userspace vs kernel

[Ingo Molnar]

* Oren Laadan <orenl at cs.columbia.edu> wrote:

> <3> Clone with pid:
> 
> To restart processes from userspace, there needs to be a way to 
> request a specific pid--in the current pid_ns--for the child 
> process (clearly, if it isn't in use).
> 
> Why is it a disadvantage ?  to Linus, a syscall clone_with_pid() 
> "sounds like a _wonderful_ attack vector against badly written 
> user-land software...".  Actually, getting a specific pid is 
> possible without this syscall.  But the point is that it's 
> undesirable to have this functionality unrestricted.

The point is that there's a class of a difference between a racy and 
unreliable method of 'create tens of thousands of tasks to steal the 
right PID you are interested in' and a built-in syscall that gives 
this within a couple of microseconds.

Most signal races are timing dependent so the ability to do it 
really quickly makes or breaks the practicality of many classes of 
exploits.

> So one option is to require root privileges. Another option is to 
> restrict such action in pid_ns created by the same user. Even more 
> so, restrict to only containers that are being restarted.

Requiring root privileges seems to remove much of the appeal of 
allowing this to be a more generic sub-container creation thing. If 
regular unprivileged apps cannot use this to save/restore their own 
local task hierarchy, the whole thing becomes rather pointless, 
right?

	Ingo