Re: [PATCH] mm: workingset: fix NULL ptr dereference

[Minchan Kim <minchan@kernel.org>]

On Sun, Apr 08, 2018 at 07:49:25PM -0700, Matthew Wilcox wrote:
> On Mon, Apr 09, 2018 at 10:58:15AM +0900, Minchan Kim wrote:
> > It assumes shadow entry of radix tree relies on the init state
> > that node->private_list allocated should be list_empty state.
> > Currently, it's initailized in SLAB constructor which means
> > node of radix tree would be initialized only when *slub allocates
> > new page*, not *new object*. So, if some FS or subsystem pass
> > gfp_mask to __GFP_ZERO, slub allocator will do memset blindly.
> 
> Wait, what?  Who's declaring their radix tree with GFP_ZERO flags?
> I don't see anyone using INIT_RADIX_TREE or RADIX_TREE or RADIX_TREE_INIT
> with GFP_ZERO.

Look at fs/f2fs/inode.c
mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);

__add_to_page_cache_locked
  radix_tree_maybe_preload

add_to_page_cache_lru

What's the wrong with setting __GFP_ZERO with mapping->gfp_mask?

> 
> Although, even if nobody's doing that intentionally, if somebody has
> a bitflip with the __GFP_ZERO bit, it's going to propagate widely.
> I think something like this might be appropriate:
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 9e1100f9298f..0f55f0a0dcaa 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2714,8 +2714,10 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>  		stat(s, ALLOC_FASTPATH);
>  	}
>  
> -	if (unlikely(gfpflags & __GFP_ZERO) && object)
> -		memset(object, 0, s->object_size);
> +	if (unlikely(gfpflags & __GFP_ZERO) && object) {
> +		if (!WARN_ON_ONCE(s->ctor))
> +			memset(object, 0, s->object_size);
> +	}


>  
>  	slab_post_alloc_hook(s, gfpflags, 1, &object);
>  
> 
> Something you could try is checking that the list is empty when the node
> is inserted into the radix tree.
> 
> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index 8e00138d593f..580f52d0c072 100644
> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -428,6 +428,7 @@ radix_tree_node_alloc(gfp_t gfp_mask, struct radix_tree_node *parent,
>  		ret->exceptional = exceptional;
>  		ret->parent = parent;
>  		ret->root = root;
> +		BUG_ON(!list_empty(&ret->private_list));
>  	}
>  	return ret;
>  }