Re: [PATCH V2] hfs: do not free node before using

[Viacheslav Dubeyko <slava@dubeyko.com>]

On Sat, 2018-11-24 at 10:10 +0800, Pan Bian wrote:
> The function hfs_bmap_free frees node via hfs_bnode_put(node).
> However,
> it then reads node->this when dumping error message on an error path,
> which may result in a use-after-free bug. This patch frees node only
> when it is never used.
> 
> Fixes: a1185ffa2fc("HFS rewrite")
> 
> Signed-off-by: Pan Bian <bianpan2016@163.com>
> 
> ---
> V2: correct the commit information in Fixes


By the way, HFS+ has the same issue [1].

> 
> ---
>  fs/hfs/btree.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 98b96ff..19017d2 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -338,13 +338,14 @@ void hfs_bmap_free(struct hfs_bnode *node)
>  
>  		nidx -= len * 8;
>  		i = node->next;
> -		hfs_bnode_put(node);
>  		if (!i) {
>  			/* panic */;
>  			pr_crit("unable to free bnode %u. bmap not
> found!\n",
>  				node->this);
> +			hfs_bnode_put(node);
>  			return;
>  		}
> +		hfs_bnode_put(node);

First of all, it looks weird to have two calls of hfs_bnode_put(node).
Secondly, you will add only one line of code instead of two if you
simply save the node->this in the local variable.

>  		node = hfs_bnode_find(tree, i);
>  		if (IS_ERR(node))
>  			return;


Thanks,
Vyacheslav Dubeyko.

[1] https://elixir.bootlin.com/linux/latest/source/fs/hfsplus/btree.c#L457