tcp: fix out-of-bounds access in ULP sysctl

KASAN reports out-of-bound access in proc_dostring() coming from
proc_tcp_available_ulp() because in case TCP ULP list is empty
the buffer allocated for the response will not have anything
printed into it.  Set the first byte to zero to avoid strlen()
going out-of-bounds.

Fixes: 734942cc4ea6 ("tcp: ULP infrastructure")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_ulp.c b/net/ipv4/tcp_ulp.c
index e855ea70819b6eb9e144945ea65388e98f9a08ac..2417f55374c593c89b2aeb1ec4f6e6e74bb1395f 100644 (file)
--- a/net/ipv4/tcp_ulp.c
+++ b/net/ipv4/tcp_ulp.c
@@ -88,6 +88,7 @@ void tcp_get_available_ulp(char *buf, size_t maxlen)
        struct tcp_ulp_ops *ulp_ops;
        size_t offs = 0;
 
+       *buf = '\0';
        rcu_read_lock();
        list_for_each_entry_rcu(ulp_ops, &tcp_ulp_list, list) {
                offs += snprintf(buf + offs, maxlen - offs,