Re: [PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item

From: Andrea Arcangeli
Date: Thu May 05 2016 - 17:57:40 EST

Hello Zhou,

Great catch.

On Thu, May 05, 2016 at 08:42:56PM +0800, Zhou Chengming wrote:
> remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
> + up_read(&mm->mmap_sem);
> spin_lock(&ksm_mmlist_lock);
> ksm_scan.mm_slot = list_entry(slot->,
> @@ -1666,16 +1667,12 @@ next_mm:
> */
> hash_del(&slot->link);
> list_del(&slot->mm_list);
> - spin_unlock(&ksm_mmlist_lock);
> free_mm_slot(slot);
> clear_bit(MMF_VM_MERGEABLE, &mm->flags);
> - up_read(&mm->mmap_sem);
> mmdrop(mm);

I thought the mmap_sem for reading prevented a race of the above
clear_bit against a concurrent madvise(MADV_MERGEABLE) which takes the
mmap_sem for writing. After this change can't __ksm_enter run
concurrently with the clear_bit above introducing a different SMP race

> - } else {
> - spin_unlock(&ksm_mmlist_lock);
> - up_read(&mm->mmap_sem);

The strict obviously safe fix is just to invert the above two,
up_read; spin_unlock.

Then I found another instance of this same SMP race condition in
unmerge_and_remove_all_rmap_items() that you didn't fix.

Actually for the other instance of the bug the implementation above
that releases the mmap_sem early sounds safe, because it's a
ksm_text_exit that takes the clear_bit path, not just the fact we
didn't find a vma with VM_MERGEABLE set and we garbage collect the
mm_slot, while the "mm" may still alive. In the other case the "mm"
isn't alive anymore so the race with MADV_MERGEABLE shouldn't be
possible to materialize.

Could you fix it by just inverting the up_read/spin_unlock order, in
the place you patched, and add this comment:

} else {
* up_read(&mm->mmap_sem) first because after
* spin_unlock(&ksm_mmlist_lock) run, the "mm" may
* already have been freed under us by __ksm_exit()
* because the "mm_slot" is still hashed and
* ksm_scan.mm_slot doesn't point to it anymore.

And in unmerge_and_remove_all_rmap_items() same thing, except there
you can apply your up_read() early and you can just drop the "else"