[PATCH] btrfs: Fix btrfs_block_group refcnt leak

[PATCH] btrfs: Fix btrfs_block_group refcnt leak

[Xiyu Yang]

btrfs_remove_block_group() invokes btrfs_lookup_block_group(), which
returns a local reference of the blcok group that contains the given
bytenr to "block_group" with increased refcount.

When btrfs_remove_block_group() returns, "block_group" becomes invalid,
so the refcount should be decreased to keep refcount balanced.

The reference counting issue happens in several exception handling paths
of btrfs_remove_block_group(). When those error scenarios occur such as
btrfs_alloc_path() returns NULL, the function forgets to decrease its
refcnt increased by btrfs_lookup_block_group() and will cause a refcnt
leak.

Fix this issue by jumping to "out_put_group" label and calling
btrfs_put_block_group() when those error scenarios occur.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
---
 fs/btrfs/block-group.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 404e050ce8ee..d9f432bd3329 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -916,7 +916,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 	path = btrfs_alloc_path();
 	if (!path) {
 		ret = -ENOMEM;
-		goto out;
+		goto out_put_group;
 	}
 
 	/*
@@ -954,7 +954,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 		ret = btrfs_orphan_add(trans, BTRFS_I(inode));
 		if (ret) {
 			btrfs_add_delayed_iput(inode);
-			goto out;
+			goto out_put_group;
 		}
 		clear_nlink(inode);
 		/* One for the block groups ref */
@@ -977,13 +977,13 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 
 	ret = btrfs_search_slot(trans, tree_root, &key, path, -1, 1);
 	if (ret < 0)
-		goto out;
+		goto out_put_group;
 	if (ret > 0)
 		btrfs_release_path(path);
 	if (ret == 0) {
 		ret = btrfs_del_item(trans, tree_root, path);
 		if (ret)
-			goto out;
+			goto out_put_group;
 		btrfs_release_path(path);
 	}
 
@@ -1102,7 +1102,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 
 	ret = remove_block_group_free_space(trans, block_group);
 	if (ret)
-		goto out;
+		goto out_put_group;
 
 	btrfs_put_block_group(block_group);
 	btrfs_put_block_group(block_group);
@@ -1132,6 +1132,9 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 		btrfs_delayed_refs_rsv_release(fs_info, 1);
 	btrfs_free_path(path);
 	return ret;
+out_put_group:
+	btrfs_put_block_group(block_group);
+	goto out;
 }
 
 struct btrfs_trans_handle *btrfs_start_trans_remove_block_group(
-- 
2.7.4

Re: [PATCH] btrfs: Fix btrfs_block_group refcnt leak

[Filipe Manana]

On Mon, Apr 20, 2020 at 6:48 AM Xiyu Yang <xiyuyang19@fudan.edu.cn> wrote:
>
> btrfs_remove_block_group() invokes btrfs_lookup_block_group(), which
> returns a local reference of the blcok group that contains the given
> bytenr to "block_group" with increased refcount.
>
> When btrfs_remove_block_group() returns, "block_group" becomes invalid,
> so the refcount should be decreased to keep refcount balanced.
>
> The reference counting issue happens in several exception handling paths
> of btrfs_remove_block_group(). When those error scenarios occur such as
> btrfs_alloc_path() returns NULL, the function forgets to decrease its
> refcnt increased by btrfs_lookup_block_group() and will cause a refcnt
> leak.
>
> Fix this issue by jumping to "out_put_group" label and calling
> btrfs_put_block_group() when those error scenarios occur.
>
> Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>

Seems correct to me.
I would change the subject to something more clear like: "btrfs: fix
block group leak after failure to remove it"

One more suggestion below.

> ---
>  fs/btrfs/block-group.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 404e050ce8ee..d9f432bd3329 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -916,7 +916,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>         path = btrfs_alloc_path();
>         if (!path) {
>                 ret = -ENOMEM;
> -               goto out;
> +               goto out_put_group;
>         }
>
>         /*
> @@ -954,7 +954,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>                 ret = btrfs_orphan_add(trans, BTRFS_I(inode));
>                 if (ret) {
>                         btrfs_add_delayed_iput(inode);
> -                       goto out;
> +                       goto out_put_group;
>                 }
>                 clear_nlink(inode);
>                 /* One for the block groups ref */
> @@ -977,13 +977,13 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>
>         ret = btrfs_search_slot(trans, tree_root, &key, path, -1, 1);
>         if (ret < 0)
> -               goto out;
> +               goto out_put_group;
>         if (ret > 0)
>                 btrfs_release_path(path);
>         if (ret == 0) {
>                 ret = btrfs_del_item(trans, tree_root, path);
>                 if (ret)
> -                       goto out;
> +                       goto out_put_group;
>                 btrfs_release_path(path);
>         }
>
> @@ -1102,7 +1102,7 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>
>         ret = remove_block_group_free_space(trans, block_group);
>         if (ret)
> -               goto out;
> +               goto out_put_group;
>
>         btrfs_put_block_group(block_group);
>         btrfs_put_block_group(block_group);
> @@ -1132,6 +1132,9 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>                 btrfs_delayed_refs_rsv_release(fs_info, 1);
>         btrfs_free_path(path);
>         return ret;
> +out_put_group:
> +       btrfs_put_block_group(block_group);
> +       goto out;

Instead of this double goto, which tends to be error prone and harder to follow,
I suggest placing a call to btrfs_put_block_group() right after the
'out' label, with a comment
above it saying something like "once for the lookup reference" and
removing one of the btrfs_put_block_group()
calls right after calling remove_block_group_free_space(), and leaving
a comment above the other one saying "once for the block groups
rbtree".

Thanks.

>  }
>
>  struct btrfs_trans_handle *btrfs_start_trans_remove_block_group(
> --
> 2.7.4
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

Re: [PATCH] btrfs: Fix btrfs_block_group refcnt leak

[David Sterba]

On Mon, Apr 20, 2020 at 01:38:40PM +0800, Xiyu Yang wrote:
> btrfs_remove_block_group() invokes btrfs_lookup_block_group(), which
> returns a local reference of the blcok group that contains the given
> bytenr to "block_group" with increased refcount.
> 
> When btrfs_remove_block_group() returns, "block_group" becomes invalid,
> so the refcount should be decreased to keep refcount balanced.
> 
> The reference counting issue happens in several exception handling paths
> of btrfs_remove_block_group(). When those error scenarios occur such as
> btrfs_alloc_path() returns NULL, the function forgets to decrease its
> refcnt increased by btrfs_lookup_block_group() and will cause a refcnt
> leak.
> 
> Fix this issue by jumping to "out_put_group" label and calling
> btrfs_put_block_group() when those error scenarios occur.
> 
> Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>

Thanks for the fix. May I ask if this was found by code inspection or by
some analysis tool?

> @@ -1132,6 +1132,9 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>  		btrfs_delayed_refs_rsv_release(fs_info, 1);
>  	btrfs_free_path(path);
>  	return ret;
> +out_put_group:
> +	btrfs_put_block_group(block_group);
> +	goto out;

As Filipe noted, the trailing gotos are not a great pattern, what we'd
like to do is a more linear sequence where the resource
allocation/freeing nesting is obvious, like:


	x = allocate(X);
	if (!x)
		goto out;
	...
	y = allocate(Y);
	if (!y)
		goto out_free_x;
	z = allocate(Z);
	if (!z)
		goto out_free_y;
	...
	free(z);
out_free_y:
	free(y);
out_free_x:
	free(x);
out:
	return;


(where allocate/free can be refcount inc/dec and similar). Sometimes
it's not that straightforward and the freeing block needs conditionals,
but from code reading perspective this is still better than potentially
wild gotos.

Re: [PATCH] btrfs: Fix btrfs_block_group refcnt leak

[Xiyu Yang]

On Tue, Apr 21, 2020 at 12:43:15AM +0200, David Sterba wrote:
> On Mon, Apr 20, 2020 at 01:38:40PM +0800, Xiyu Yang wrote:
> > btrfs_remove_block_group() invokes btrfs_lookup_block_group(), which
> > returns a local reference of the blcok group that contains the given
> > bytenr to "block_group" with increased refcount.
> > 
> > When btrfs_remove_block_group() returns, "block_group" becomes invalid,
> > so the refcount should be decreased to keep refcount balanced.
> > 
> > The reference counting issue happens in several exception handling paths
> > of btrfs_remove_block_group(). When those error scenarios occur such as
> > btrfs_alloc_path() returns NULL, the function forgets to decrease its
> > refcnt increased by btrfs_lookup_block_group() and will cause a refcnt
> > leak.
> > 
> > Fix this issue by jumping to "out_put_group" label and calling
> > btrfs_put_block_group() when those error scenarios occur.
> > 
> > Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
> > Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
> 
> Thanks for the fix. May I ask if this was found by code inspection or by
> some analysis tool?

Thanks for your advice about the patch! We are looking for some automated ways
to find this kind of bug.