Messages in this thread | | | Date | Tue, 4 Mar 2014 20:03:48 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC] [PATCH] Pre-emption control for userspace |
| |
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.
>>> + if (current->sched_preempt_delay.mmap_state != NULL) { >>> + retval = -EEXIST; >>> + goto error; >> >> This assumes that we are going to setup current->sched_preempt_delay.mmap_state, >> but what if the task opens /proc/random_tid/sched_preempt_delay ? > > Good point. A thread should not be allowed to request preemption delay > for another thread. I would recommend leaving this code alone and adding > following code before this: > > if (get_proc_task(inode) != current) { > retval = -EPERM; > goto error; > } > > Sounds reasonable?
Yes, we should check == current, but this interface looks strange anyway, imho.
>>> + 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.
>>> + 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.
Oleg.
| |