mptcp: fix use-after-free for ipv6

Turns out that when we accept a new subflow, the newly created
inet_sk(tcp_sk)->pinet6 points at the ipv6_pinfo structure of the
listener socket.

This wasn't caught by the selftest because it closes the accepted fd
before the listening one.

adding a close(listenfd) after accept returns is enough:
 BUG: KASAN: use-after-free in inet6_getname+0x6ba/0x790
 Read of size 1 at addr ffff88810e310866 by task mptcp_connect/2518
 Call Trace:
  inet6_getname+0x6ba/0x790
  __sys_getpeername+0x10b/0x250
  __x64_sys_getpeername+0x6f/0xb0

also alter test program to exercise this.

Reported-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 0 deletions

diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index a3dccd816ae4..99579c0223c1 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
@@ -634,6 +634,14 @@ static void check_getpeername_connect(int fd)
 			cfg_host, a, cfg_port, b);
 }
 
+static void maybe_close(int fd)
+{
+	unsigned int r = rand();
+
+	if (r & 1)
+		close(fd);
+}
+
 int main_loop_s(int listensock)
 {
 	struct sockaddr_storage ss;
@@ -657,6 +665,7 @@ int main_loop_s(int listensock)
 	salen = sizeof(ss);
 	remotesock = accept(listensock, (struct sockaddr *)&ss, &salen);
 	if (remotesock >= 0) {
+		maybe_close(listensock);
 		check_sockaddr(pf, &ss, salen);
 		check_getpeername(remotesock, &ss, salen);