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

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Date:   Fri, 23 Nov 2018 04:59:53 -0800
From:   Joe Perches <joe@...ches.com>
To:     Pan Bian <bianpan2016@....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>")

Hi.

While this may indeed be a defect, and the "/* panic */" comment
may be unwarranted, this isn't really a fix of a printk conversion.

This dereference goes back to 2004, the printk(KERN_CRIT to pr_crit(
conversion did not introduce it.

So this patch is only a possible use after free fix.

>From a full history git tree:
(similar to https://archive.org/details/git-history-of-linux)

commit a1185ffa2fc491e23f3107a39f66ee703d102153
Author: Andrew Morton <akpm@...l.org>
Date:   Wed Feb 25 16:17:36 2004 -0800

    [PATCH] HFS rewrite
    
    From: Roman Zippel <zippel@...ux-m68k.org>
    
    This is a complete rewrite of the HFS driver, it gets rid of a all the
    special conversion options, which belong in user space.  The driver uses now
    a btree support very similiar to HFS+, so that both could be merged at some
    point.
    
    Thanks to Ethan Benson <erbenson@...ska.net> for a number of patches to make
    the driver more compliant with the spec and Christoph Hellwig <hch@....de>
    for fixing up the documentation.

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