linux-kernel - [PATCH bpf-next] xdp: Fix race in dev_map_hash_update

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Date:   Sun,  8 Sep 2019 09:20:16 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     make-wifi-fast@...ts.bufferbloat.net,
        linux-wireless@...r.kernel.org, ast@...nel.org,
        bpf@...r.kernel.org, daniel@...earbox.net, davem@...emloft.net,
        hawk@...nel.org, jakub.kicinski@...ronome.com,
        john.fastabend@...il.com, kafai@...com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        songliubraving@...com, syzkaller-bugs@...glegroups.com, yhs@...com
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        syzbot+4e7a85b1432052e8d6f8@...kaller.appspotmail.com
Subject: [PATCH bpf-next] xdp: Fix race in dev_map_hash_update_elem() when replacing element

syzbot found a crash in dev_map_hash_update_elem(), when replacing an
element with a new one. Jesper correctly identified the cause of the crash
as a race condition between the initial lookup in the map (which is done
before taking the lock), and the removal of the old element.

Rather than just add a second lookup into the hashmap after taking the
lock, fix this by reworking the function logic to take the lock before the
initial lookup.

Fixes: 6f9d451ab1a3 ("xdp: Add devmap_hash map type for looking up devices by hashed index")
Reported-and-tested-by: syzbot+4e7a85b1432052e8d6f8@...kaller.appspotmail.com
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 kernel/bpf/devmap.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 9af048a932b5..d27f3b60ff6d 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -650,19 +650,22 @@ static int __dev_map_hash_update_elem(struct net *net, struct bpf_map *map,
 	u32 ifindex = *(u32 *)value;
 	u32 idx = *(u32 *)key;
 	unsigned long flags;
+	int err = -EEXIST;
 
 	if (unlikely(map_flags > BPF_EXIST || !ifindex))
 		return -EINVAL;
 
+	spin_lock_irqsave(&dtab->index_lock, flags);
+
 	old_dev = __dev_map_hash_lookup_elem(map, idx);
 	if (old_dev && (map_flags & BPF_NOEXIST))
-		return -EEXIST;
+		goto out_err;
 
 	dev = __dev_map_alloc_node(net, dtab, ifindex, idx);
-	if (IS_ERR(dev))
-		return PTR_ERR(dev);
-
-	spin_lock_irqsave(&dtab->index_lock, flags);
+	if (IS_ERR(dev)) {
+		err = PTR_ERR(dev);
+		goto out_err;
+	}
 
 	if (old_dev) {
 		hlist_del_rcu(&old_dev->index_hlist);
@@ -683,6 +686,10 @@ static int __dev_map_hash_update_elem(struct net *net, struct bpf_map *map,
 		call_rcu(&old_dev->rcu, __dev_map_entry_free);
 
 	return 0;
+
+out_err:
+	spin_unlock_irqrestore(&dtab->index_lock, flags);
+	return err;
 }
 
 static int dev_map_hash_update_elem(struct bpf_map *map, void *key, void *value,
-- 
2.23.0