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

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Date:   Fri, 23 Nov 2018 16:51:49 -0800
From:   Viacheslav Dubeyko <slava@...eyko.com>
To:     Pan Bian <bianpan2016@....com>, Joe Perches <joe@...ches.com>,
        "Ernesto A." Fernández 
        <ernesto.mnd.fernandez@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hfs: do not free node before using

On Fri, 2018-11-23 at 17:04 +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: d614267329f("hfs/hfsplus: convert printks to pr_<level>")
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>
> ---
>  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);


What's about simply to store node->this into the local variable in the
beginning of the cycle? In this case, it is possible to use the node ID
in the pr_crit() and to use the hfs_bnode_put(node) in the same place
without to use it twice. What do you think?

Thanks,
Vyacheslav Dubeyko.


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