[PATCH net-next 3/4] ethtool: strset: check nla_len overflow before nla_nest_end

[Hangbin Liu <liuhangbin@gmail.com>]

The netlink attribute length field nla_len is a __u16, which can only
represent values up to 65535 bytes. NICs with a large number of
statistics strings (e.g. mlx5_core with thousands of ETH_SS_STATS
entries) can produce a ETHTOOL_A_STRINGSET_STRINGS nest that exceeds
this limit.

When nla_nest_end() writes the actual nest size back to nla_len, the
value is silently truncated. This results in a corrupted netlink message
being sent to userspace: the parser reads a wrong (truncated) attribute
length and misaligns all subsequent attribute boundaries, causing decode
errors.

Fix this by checking whether the size of strings_attr would exceed
U16_MAX after all strings have been written, and give up nla put if so.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
---
 net/ethtool/strset.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ethtool/strset.c b/net/ethtool/strset.c
index f6a67109beda..9c502b290f5c 100644
--- a/net/ethtool/strset.c
+++ b/net/ethtool/strset.c
@@ -441,6 +441,10 @@ static int strset_fill_set(struct sk_buff *skb,
 			if (strset_fill_string(skb, set_info, i) < 0)
 				goto nla_put_failure;
 		}
+
+		if (skb_tail_pointer(skb) - (unsigned char *)strings_attr > U16_MAX)
+			goto nla_put_failure;
+
 		nla_nest_end(skb, strings_attr);
 	}
 

-- 
Git-155)