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

[Joe Perches]

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 <a...@osdl.org>
Date:   Wed Feb 25 16:17:36 2004 -0800

    [PATCH] HFS rewrite
    
    From: Roman Zippel <zip...@linux-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 <erben...@alaska.net> for a number of patches to make
    the driver more compliant with the spec and Christoph Hellwig <h...@lst.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;