[PATCH net 4/4] drop_monitor: fix out-of-bounds write in reset_per_cpu_data()

[Eric Dumazet <edumazet@google.com>]

In reset_per_cpu_data(), al is computed as:

    al = sizeof(struct net_dm_alert_msg);
    al += dm_hit_limit * sizeof(struct net_dm_drop_point);
    al += sizeof(struct nlattr);

    skb = genlmsg_new(al, GFP_KERNEL);
    ...
    nla = nla_reserve(skb, NLA_UNSPEC, sizeof(struct net_dm_alert_msg));
    ...
    msg = nla_data(nla);
    memset(msg, 0, al);

Because al includes sizeof(struct nlattr) (the 4-byte attribute header),
genlmsg_new() allocates al bytes of tailroom starting at nla.
However, msg points to nla_data(nla), which is located
sizeof(struct nlattr) bytes past nla. Calling memset(msg, 0, al)
therefore writes al bytes starting from msg, exceeding the allocated
buffer by sizeof(struct nlattr) (4 bytes) and corrupting
skb_shared_info.

Fix this by letting al represent only the payload length, allocating
the skb with genlmsg_new(nla_total_size(al), GFP_KERNEL), and zeroing
al bytes from msg.

Fixes: 683703a26e46 ("drop_monitor: Update netlink protocol to include netlink attribute header in alert message")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/drop_monitor.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index 795c15dd1771a2a5f15e109d0ef2eed967c45443..edc660778408e1bb996124be5a5349dfc9be3568 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c
@@ -141,9 +141,8 @@ static struct sk_buff *reset_per_cpu_data(struct per_cpu_dm_data *data)
 
 	al = sizeof(struct net_dm_alert_msg);
 	al += dm_hit_limit * sizeof(struct net_dm_drop_point);
-	al += sizeof(struct nlattr);
 
-	skb = genlmsg_new(al, GFP_KERNEL);
+	skb = genlmsg_new(nla_total_size(al), GFP_KERNEL);
 
 	if (!skb)
 		goto err;
-- 
2.55.0.1007.g17ff1f9808-goog