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

[Michal Hocko]

On Mon 09-04-18 10:58:15, Minchan Kim wrote:
> Recently, I got a report like below.
> 
> [ 7858.792946] [<ffffff80086f4de0>] __list_del_entry+0x30/0xd0
> [ 7858.792951] [<ffffff8008362018>] list_lru_del+0xac/0x1ac
> [ 7858.792957] [<ffffff800830f04c>] page_cache_tree_insert+0xd8/0x110
> [ 7858.792962] [<ffffff8008310188>] __add_to_page_cache_locked+0xf8/0x4e0
> [ 7858.792967] [<ffffff800830ff34>] add_to_page_cache_lru+0x50/0x1ac
> [ 7858.792972] [<ffffff800830fdd0>] pagecache_get_page+0x468/0x57c
> [ 7858.792979] [<ffffff80085d081c>] __get_node_page+0x84/0x764
> [ 7858.792986] [<ffffff800859cd94>] f2fs_iget+0x264/0xdc8
> [ 7858.792991] [<ffffff800859ee00>] f2fs_lookup+0x3b4/0x660
> [ 7858.792998] [<ffffff80083d2540>] lookup_slow+0x1e4/0x348
> [ 7858.793003] [<ffffff80083d0eb8>] walk_component+0x21c/0x320
> [ 7858.793008] [<ffffff80083d0010>] path_lookupat+0x90/0x1bc
> [ 7858.793013] [<ffffff80083cfe6c>] filename_lookup+0x8c/0x1a0
> [ 7858.793018] [<ffffff80083c52d0>] vfs_fstatat+0x84/0x10c
> [ 7858.793023] [<ffffff80083c5b00>] SyS_newfstatat+0x28/0x64
> 
> v4.9 kenrel already has the d3798ae8c6f3,("mm: filemap: don't
> plant shadow entries without radix tree node") so I thought
> it should be okay. When I was googling, I found others report
> such problem and I think current kernel still has the problem.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1431567
> https://bugzilla.redhat.com/show_bug.cgi?id=1420335
> 
> 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.
> That means allocated node can have !list_empty(node->private_list).
> It ends up calling NULL deference at workingset_update_node by
> failing list_empty check.
> 
> This patch should fix it.
> 
> Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
> Reported-by: Chris Fries <cfries@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Regardless of whether it makes sense to use __GFP_ZERO from the upper
layer or not, it is subtle as hell to rely on the pre-existing state
for a newly allocated object. So yes this makes perfect sense.

Do we want CC: stable?
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> If it is reviewed and proved with testing, I will resend the patch to
> Ccing stable@xxxxxxxxxxxxxxxx
> 
> Thanks.
> 
>  lib/radix-tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index 8e00138d593f..afcbdb6c495f 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;
> +		INIT_LIST_HEAD(&ret->private_list);
>  	}
>  	return ret;
>  }
> @@ -2234,7 +2235,6 @@ radix_tree_node_ctor(void *arg)
>  	struct radix_tree_node *node = arg;
>  
>  	memset(node, 0, sizeof(*node));
> -	INIT_LIST_HEAD(&node->private_list);
>  }
>  
>  static __init unsigned long __maxindex(unsigned int height)
> -- 
> 2.17.0.484.g0c8726318c-goog

-- 
Michal Hocko
SUSE Labs