linux-kernel - Re: [PATCH] mm: workingset: fix NULL ptr dereference

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Message-ID: <20180409024925.GA21889@bombadil.infradead.org>
Date:   Sun, 8 Apr 2018 19:49:25 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Christopher Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        Chris Fries <cfries@...gle.com>
Subject: Re: [PATCH] mm: workingset: fix NULL ptr dereference

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.

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;
 }