Messages in this thread | | | Date | Tue, 04 Mar 2014 13:14:51 -0700 | From | Khalid Aziz <> | Subject | Re: [RFC] [PATCH] Pre-emption control for userspace |
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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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