Re: [kvm-devel] [RFC/PATCH 01/15] preparation: provide hook to enable pgstes in user pagetable

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> What you've done with dup_mm() is probably the brute-force way that I
> would have done it had I just been trying to make a proof of concept or
> something.  I'm worried that there are a bunch of corner cases that
> haven't been considered.
> 
> What if someone else is poking around with ptrace or something similar
> and they bump the mm_users:
> 
> +       if (tsk->mm->context.pgstes)
> +               return 0;
> +       if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 ||
> +           tsk->mm != tsk->active_mm || tsk->mm->ioctx_list)
> +               return -EINVAL;
> -------->HERE
> +       tsk->mm->context.pgstes = 1;    /* dirty little tricks .. */
> +       mm = dup_mm(tsk);
> 
> It'll race, possibly fault in some other pages, and those faults will be
> lost during the dup_mm().  I think you need to be able to lock out all
> of the users of access_process_vm() before you go and do this.  You also
> need to make sure that anyone who has looked at task->mm doesn't go and
> get a reference to it and get confused later when it isn't the task->mm
> any more.
> 
> > Therefore, we need to reallocate the page table after fork() 
> > once we know that task is going to be a hypervisor. That's what this 
> > code does: reallocate a bigger page table to accomondate the extra 
> > information. The task needs to be single-threaded when calling for 
> > extended page tables.
> > 
> > Btw: at fork() time, we cannot tell whether or not the user's going to 
> > be a hypervisor. Therefore we cannot do this in fork.
> 
> Can you convert the page tables at a later time without doing a
> wholesale replacement of the mm?  It should be a bit easier to keep
> people off the pagetables than keep their grubby mitts off the mm
> itself.

Yes, as far as I can see you're right. And whatever we do in arch code,
after all it's just a work around to avoid a new clone flag.
If something like clone() with CLONE_KVM would be useful for more
architectures than just s390 then maybe we should try to get a flag.

Oh... there are just two unused clone flag bits left. Looks like the
namespace changes ate up a lot of them lately.

Well, we could still play dirty tricks like setting a bit in current
via whatever mechanism which indicates child-wants-extended-page-tables
and then just fork and be happy.

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