2018-11-24 08:57:33

by Pan Bian

Subject: [PATCH V2] hfs: do not free node before using

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 <[email protected]>

---
V2: correct the commit information in Fixes

---
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);
node = hfs_bnode_find(tree, i);
if (IS_ERR(node))
return;
--
2.7.4

2018-11-24 20:05:14

by Viacheslav Dubeyko

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 <[email protected]>
>
> ---
> 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