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

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Date:   Sat, 24 Nov 2018 09:54:23 +0800
From:   PanBian <bianpan2016@....com>
To:     Viacheslav Dubeyko <slava@...eyko.com>
Cc:     Joe Perches <joe@...ches.com>,
        Ernesto A. Fernández 
        <ernesto.mnd.fernandez@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hfs: do not free node before using

On Fri, Nov 23, 2018 at 04:51:49PM -0800, Viacheslav Dubeyko wrote:
> 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?

Of course, it is a solution. But I think doing like that does not really
simplify the code, as we have to declare an extra local variable, and
assign node->this to it...

Best,
Pan

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