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SubjectRe: [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 (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()?


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.


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