Re: [RFC] [PATCH] Pre-emption control for userspace

[Khalid Aziz]

On 03/04/2014 12:03 PM, Oleg Nesterov wrote:
> On 03/04, Khalid Aziz wrote:
> > On 03/04/2014 06:56 AM, Oleg Nesterov wrote:
> > > Hmm. In fact I think do_exit() should crash after munmap? ->mmap_state
> > > should be NULL ?? Perhaps I misread this patch completely...
> >
> > do_exit() unmaps mmap_state->uaddr, and frees up mmap_state->kaddr and
> > mmap_state. mmap_state should not be NULL after unmap.
>
> Can't understand... do_exit() does:
>
> 	+#if CONFIG_SCHED_PREEMPT_DELAY
> 	+       if (tsk->sched_preempt_delay.mmap_state) {
> 	+               sys_munmap((unsigned long)
> 	+                       tsk->sched_preempt_delay.mmap_state->uaddr, PAGE_SIZE);
> 	+               vfree(tsk->sched_preempt_delay.mmap_state->kaddr);
> 	+               kfree(tsk->sched_preempt_delay.mmap_state);
> 	
> sys_munmap() (which btw should not be used) obviously unmaps that
> vma and vma_ops()->close() should be called.
>
> close_preempt_delay_vmops() does:
>
> 	state->task->sched_preempt_delay.mmap_state = NULL;
>
> vfree(tsk->sched_preempt_delay.mmap_state->kaddr) above will try to
> dereference .mmap_state == NULL.
>
> IOW, I think that with this patch this trivial program
>
> 	int main(void)
> 	{
> 		fd = open("/proc/self/task/$TID/sched_preempt_delay", O_RDWR);
> 		mmap(NULL, 4096, PROT_READ,MAP_SHARED, fd, 0);
> 		return 0;
> 	}
>
> should crash the kernel.
>
> > > > +	state->page = page;
> > > > +	state->kaddr = kaddr;
> > > > +	state->uaddr = (void *)vma->vm_start;
> > >
> > > This is used by do_exit(). But ->vm_start can be changed by mremap() ?
> > >
> > >
> > > Hmm. And mremap() can do vm_ops->close() too. But the new vma will
> > > have the same vm_ops/vm_private_data, so exit_mmap() will try to do
> > > this again... Perhaps I missed something, but I bet this all can't be
> > > right.
> >
> > Would you say sys_munmap() of mmap_state->uaddr is not even needed since
> > exit_mm() will do this any way further down in do_exit()?
>
> No.
>
> I meant:
>
> 	1. mremap() can move this vma, so do_exit() can't trust ->uaddr
>
> 	2. Even worse, mremap() itself is not safe. It can do ->close()
> 	   too and create the new vma with the same vm_ops. Another
> 	   unmap from (say) exit_mm() won't be happy.

I agree this looks like a potential spot for trouble. I was asking if 
removing sys_munmap() of uaddr from do_exit() solves both of the above 
problems? You have convinced me this sys_munmap() I added is unnecessary.

> > > > +	vma->vm_flags |= VM_DONTCOPY | VM_DONTEXPAND | VM_SHARED | VM_WRITE;
> > >
> > > This probably also needs VM_IO, to protect from madvise(MADV_DOFORK).
> >
> > Yes, you are right. I will add that.
> >
> > > VM_SHARED/VM_WRITE doesn't look right.
> >
> > VM_SHARED is wrong but VM_WRITE is needed I think since the thread will
> > write to the mmap'd page to signal to request preemption delay.
>
> But ->mmap() should not set VM_WRITE if application does mmap(PROT_READ) ?
> VM_WRITE-or-not should be decided by do_mmap_pgoff/mprotect, ->mmap()
> should not play with this bit.

Ah, I see. This makes sense. I will remove it.

Thanks,
Khalid


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