[PATCH] hfs: do not free node before using

[PATCH] hfs: do not free node before using

[Pan Bian]

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@163.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);
+			hfs_bnode_put(node);
 			return;
 		}
+		hfs_bnode_put(node);
 		node = hfs_bnode_find(tree, i);
 		if (IS_ERR(node))
 			return;
-- 
2.7.4

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

    [PATCH] HFS rewrite
    
    From: Roman Zippel <zippel@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 <erbenson@alaska.net> for a number of patches to make
    the driver more compliant with the spec and Christoph Hellwig <hch@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;

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

[Viacheslav Dubeyko]

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@163.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;

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

[PanBian]

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@163.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;