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

[Andrea Arcangeli <aarcange@redhat.com>]

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->mm_list.next,
> @@ -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
condition?

> -	} 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.
		 */
		up_read(&mm->mmap_sem);
		spin_unlock(&ksm_mmlist_lock);
	}

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"
clause.

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