[PATCH v2] net/ipv4/tcp_cong: Replace strncpy() with strscpy()

[Kees Cook]

Replace the deprecated[1] uses of strncpy() in tcp_ca_get_name_by_key()
and tcp_get_default_congestion_control(). The callers use the results as
standard C strings (via nla_put_string() and proc handlers respectively),
so trailing padding is not needed.

Since passing the destination buffer arguments decays it to a pointer,
the size can't be trivially determined by the compiler. ca->name is
the same length in both cases, so strscpy() won't fail (when ca->name
is NUL-terminated). Include the length explicitly instead of using the
2-argument strscpy().

Link: https://github.com/KSPP/linux/issues/90 [1]
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
 v2: add tcp_get_default_congestion_control() conversion
 v1: https://lore.kernel.org/lkml/20240711171652.work.887-kees@xxxxxxxxxx/
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
---
 net/ipv4/tcp_cong.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 28ffcfbeef14..874531c7c08b 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -203,9 +203,10 @@ char *tcp_ca_get_name_by_key(u32 key, char *buffer)
 
 	rcu_read_lock();
 	ca = tcp_ca_find_key(key);
-	if (ca)
-		ret = strncpy(buffer, ca->name,
-			      TCP_CA_NAME_MAX);
+	if (ca) {
+		strscpy(buffer, ca->name, TCP_CA_NAME_MAX);
+		ret = buffer;
+	}
 	rcu_read_unlock();
 
 	return ret;
@@ -338,7 +339,7 @@ void tcp_get_default_congestion_control(struct net *net, char *name)
 
 	rcu_read_lock();
 	ca = rcu_dereference(net->ipv4.tcp_congestion_control);
-	strncpy(name, ca->name, TCP_CA_NAME_MAX);
+	strscpy(name, ca->name, TCP_CA_NAME_MAX);
 	rcu_read_unlock();
 }
 
-- 
2.34.1