From: =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
Date: Sun, 8 Sep 2019 09:20:16 +0100
Subject: xdp: Fix race in dev_map_hash_update_elem() when replacing element
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Patch-mainline: v5.4-rc1
Git-commit: af58e7ee6a8d83726ad8a2696e98d86400a7639c
References: bsc#1154353

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@syzkaller.appspotmail.com
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 kernel/bpf/devmap.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -650,19 +650,22 @@ static int __dev_map_hash_update_elem(st
 	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(st
 		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,