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

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Message-ID: <1543089821.12764.7.camel@dubeyko.com>
Date:   Sat, 24 Nov 2018 12:03:41 -0800
From:   Viacheslav Dubeyko <slava@...eyko.com>
To:     Pan Bian <bianpan2016@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        ernesto.mnd.fernandez@...il.com, Joe Perches <joe@...ches.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] hfs: do not free node before using

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@....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