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SubjectRe: Creating tasks on restart: userspace vs kernel

* Oren Laadan <orenl@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


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