Subject: Re: [PATCH] net/mlx4_en: Fix bpf_prog_add ref_cnt in mlx4

From: Daniel Borkmann <[email-masked]>
Date: Wed, 09 Nov 2016 10:57:32 +0100

On 11/09/2016 10:45 AM, Zhiyi Sun wrote:
> On Wed, Nov 09, 2016 at 10:05:31AM +0100, Daniel Borkmann wrote:
>> On 11/09/2016 08:35 AM, Zhiyi Sun wrote:
>>> There are rx_ring_num queues. Each queue will load xdp prog. So
>>> bpf_prog_add() should add rx_ring_num to ref_cnt.
>>>
>>> Signed-off-by: Zhiyi Sun <[email-masked]>
>>
>> Your analysis looks incorrect to me. Please elaborate in more detail why
>> you think current code is buggy ...
>
> Yes, you are correct. My patch is incorrect. It is not a bug.
>
>> Call path is dev_change_xdp_fd(), which does bpf_prog_get_type() on the
>> fd. This already takes a ref and only drops it in case of error. Thus
>> in mlx4_xdp_set(), you only need priv->rx_ring_num - 1 refs for the rest
>> of the rings, so that dropping refs from old_prog makes sure we release
>> it again. Looks correct to me (maybe a comment would have helped there).
>
> I thought mlx4's code is incorrect because in mlx5's driver, function
> mlx5e_xdp_set() calls a pair of bpf_prog_add/put, the number of add and
> put to the refs are same. I didn't notice that one "add" has been called in its
> calller. So, it seems that mlx5's code is incorrect, right?

Yep, I think the two attached patches are needed.

The other thing I noticed in mlx5e_create_rq() is that it calls
bpf_prog_add(rq->xdp_prog, 1) without actually checking for errors.
>>From d2bd6b3cd8636716a06b0ea3b1e041e16f87cce0 Mon Sep 17 00:00:00 2001
Message-Id: <[email-masked]>
From: Daniel Borkmann <[email-masked]>
Date: Wed, 9 Nov 2016 10:31:19 +0100
Subject: [PATCH net-next 1/2] bpf, mlx4: fix prog refcount in mlx4_en_try_alloc_resources error path

Commit 67f8b1dcb9ee ("net/mlx4_en: Refactor the XDP forwarding rings
scheme") added a bug in that the prog's reference count is not dropped
in the error path when mlx4_en_try_alloc_resources() is failing.

We previously took bpf_prog_add(prog, priv->rx_ring_num - 1), that we
need to release again. Earlier in the call-path, dev_change_xdp_fd()
itself holds a ref to the prog as well, which is then released though
bpf_prog_put() due to the propagated error.

Fixes: 67f8b1dcb9ee ("net/mlx4_en: Refactor the XDP forwarding rings scheme")
Signed-off-by: Daniel Borkmann <[email-masked]>
---
 drivers/net/ethernet/mellanox/mlx4/en_netdev.c |  5 ++++-
 include/linux/bpf.h                            |  1 +
 kernel/bpf/syscall.c                           | 11 +++++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
index 0f6225c..4104aec 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
@@ -2747,8 +2747,11 @@ static int mlx4_xdp_set(struct net_device *dev, struct bpf_prog *prog)
 	}
 
 	err = mlx4_en_try_alloc_resources(priv, tmp, &new_prof);
-	if (err)
+	if (err) {
+		if (prog)
+			bpf_prog_add_undo(prog, priv->rx_ring_num - 1);
 		goto unlock_out;
+	}
 
 	if (priv->port_up) {
 		port_up = 1;
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index edcd96d..4f6a4f1 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -234,6 +234,7 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
 struct bpf_prog *bpf_prog_get(u32 ufd);
 struct bpf_prog *bpf_prog_get_type(u32 ufd, enum bpf_prog_type type);
 struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i);
+void bpf_prog_add_undo(struct bpf_prog *prog, int i);
 struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog);
 void bpf_prog_put(struct bpf_prog *prog);
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 228f962..a6e4dd8 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -680,6 +680,17 @@ struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
 }
 EXPORT_SYMBOL_GPL(bpf_prog_add);
 
+void bpf_prog_add_undo(struct bpf_prog *prog, int i)
+{
+	/* Only to be used for undoing previous bpf_prog_add() in some
+	 * error path. We still know that another entity in our call
+	 * path holds a reference to the program, thus atomic_sub() can
+	 * be safely used here!
+	 */
+	atomic_sub(i, &prog->aux->refcnt);
+}
+EXPORT_SYMBOL_GPL(bpf_prog_add_undo);
+
 struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog)
 {
 	return bpf_prog_add(prog, 1);
-- 
1.9.3

>>From f0789544432bbb89c53c3b8ac6575d48fed97786 Mon Sep 17 00:00:00 2001
Message-Id: <[email-masked]>
In-Reply-To: <[email-masked]>
References: <[email-masked]>
From: Daniel Borkmann <[email-masked]>
Date: Wed, 9 Nov 2016 10:51:26 +0100
Subject: [PATCH net-next 2/2] bpf, mlx5: fix prog refcount in mlx5e_xdp_set

dev_change_xdp_fd() already holds a reference, so bpf_prog_add(prog, 1)
is not correct as it takes one reference too much and will thus leak
the prog eventually. Also, bpf_prog_add() can fail and is not checked
for errors here.

Fixes: 86994156c736 ("net/mlx5e: XDP fast RX drop bpf programs support")
Signed-off-by: Daniel Borkmann <[email-masked]>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
index ba0c774..63309dd 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
@@ -3121,8 +3121,6 @@ static int mlx5e_xdp_set(struct net_device *netdev, struct bpf_prog *prog)
 
 	/* exchange programs */
 	old_prog = xchg(&priv->xdp_prog, prog);
-	if (prog)
-		bpf_prog_add(prog, 1);
 	if (old_prog)
 		bpf_prog_put(old_prog);
 
-- 
1.9.3