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SubjectRe: [RFC][v8][PATCH 0/10] Implement clone3() system call
Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> writes:

> Eric W. Biederman [ebiederm@xmission.com] wrote:
> | > Could you clarify ? How is the call to alloc_pidmap() from clone3() different
> | > from the call from clone() itself ?
> |
> | I think it is totally inappropriate to assign pids in a pid namespace
> | where there are user space processes already running.
>
> Honestly, I don't understand why it is inappropriate or how this differs
> from normal clone() - which also assigns pids in own and ancestor pid
> namespaces.

The fact we can specify which pids we want. I won't claim it is as
exploitable as NULL pointer deferences have been but it has that kind
of feel to it.

> | > | How we handle a clone extension depends critically on if we want to
> | > | create a processes for restart in user space or kernel space.
> | > |
> | > | Could some one give me or point me at a strong case for creating the
> | > | processes for restart in user space?
> | >
> | > There has been a lot of discussion on this with reference to the
> | > Checkpoint/Restart patchset. See http://lkml.org/lkml/2009/4/13/401
> | > for instance.
> |
> | Just read it. Thank you.
>
> Sorry. I should have mentioned the reason here. (Like you mention below),
> flexibility is the main reason.
>
> | Now I am certain clone_with_pids() is not useful functionality to be
> | exporting to userspace.
> |
> | The only real argument in favor of doing this in user space is greater
> | flexibility. I can see checkpointing/restoring a single thread process
> | without a pid namespace. Anything more and you are just asking for
> | trouble.
> |
> | A design that weakens security. Increases maintenance costs. All for
> | an unreliable result seems like a bad one to me.
> |
> | > | The pid assignment code is currently ugly. I asked that we just pass
> | > | in the min max pid pids that already exist into the core pid
> | > | assignment function and a constrained min/max that only admits a
> | > | single pid when we are allocating a struct pid for restart. That was
> | > | not done and now we have a weird abortion with unnecessary special cases.
> | >
> | > I did post a version of the patch attemptint to implement that. As
> | > pointed out in:
> | >
> | > http://lkml.org/lkml/2009/8/17/445
> | >
> | > we would need more checks in alloc_pidmap() to cover cases like min or max
> | > being invalid or min being greater than max or max being greater than pid_max
> | > etc. Those checks also made the code ugly (imo).
> |
> | If you need more checks you are doing it wrong. The code already has min
> | and max values, and even a start value. I was just strongly suggesting
> | we generalize where we get the values from, and then we have not special
> | cases.
>
> Well, if alloc_pidmap(pid_ns, min, max) does not have to check the
> parameters passed in (ie assumes that callers pass it in correctly)
> it might be simple. But when user specifies the pid, the
>
> min == max == user's target pid
>
> so we will need to check the values either here or in callers.

Agreed. When you are talking about the target pid. That code path
needs the extra check.

> Yes the code already has values and a start value. But these are
> controlled by alloc_pidmap() and not passed in from the user space.

I was only thinking passed in from someplace else in kernel/pid.c

> alloc_pidmap() needs to assign the next available pid or a specific
> target pid. Generalizing it to alloc a pid in a range seemed be a
> bit of an over kill for currently known usages.

alloc_pidmap in assigning the next available pid is allocating a pid
in a range.

> I will post a version of the patch outside this patchset with min
> and max parameters and we can see if it can be optimized/beautified.

Thanks,
Eric



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