sctp: initialize more fields in sctp_v6_from_sk()

[ Upstream commit 2e87504 ]

syzbot found that sin6_scope_id was not properly initialized,
leading to undefined behavior.

Clear sin6_scope_id and sin6_flowinfo.

BUG: KMSAN: uninit-value in __sctp_v6_cmp_addr+0x887/0x8c0 net/sctp/ipv6.c:649
  __sctp_v6_cmp_addr+0x887/0x8c0 net/sctp/ipv6.c:649
  sctp_inet6_cmp_addr+0x4f2/0x510 net/sctp/ipv6.c:983
  sctp_bind_addr_conflict+0x22a/0x3b0 net/sctp/bind_addr.c:390
  sctp_get_port_local+0x21eb/0x2440 net/sctp/socket.c:8452
  sctp_get_port net/sctp/socket.c:8523 [inline]
  sctp_listen_start net/sctp/socket.c:8567 [inline]
  sctp_inet_listen+0x710/0xfd0 net/sctp/socket.c:8636
  __sys_listen_socket net/socket.c:1912 [inline]
  __sys_listen net/socket.c:1927 [inline]
  __do_sys_listen net/socket.c:1932 [inline]
  __se_sys_listen net/socket.c:1930 [inline]
  __x64_sys_listen+0x343/0x4c0 net/socket.c:1930
  x64_sys_call+0x271d/0x3e20 arch/x86/include/generated/asm/syscalls_64.h:51
  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
  do_syscall_64+0xd9/0x210 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Local variable addr.i.i created at:
  sctp_get_port net/sctp/socket.c:8515 [inline]
  sctp_listen_start net/sctp/socket.c:8567 [inline]
  sctp_inet_listen+0x650/0xfd0 net/sctp/socket.c:8636
  __sys_listen_socket net/socket.c:1912 [inline]
  __sys_listen net/socket.c:1927 [inline]
  __do_sys_listen net/socket.c:1932 [inline]
  __se_sys_listen net/socket.c:1930 [inline]
  __x64_sys_listen+0x343/0x4c0 net/socket.c:1930

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: syzbot+e69f06a0f30116c68056@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/68adc0a2.050a0220.37038e.00c4.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20250826141314.1802610-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: edumazet

net/sctp/ipv6.c

@@ -546,7 +546,9 @@ static void sctp_v6_from_sk(union sctp_addr *addr, struct sock *sk)
 {
 	addr->v6.sin6_family = AF_INET6;
 	addr->v6.sin6_port = 0;
+	addr->v6.sin6_flowinfo = 0;
 	addr->v6.sin6_addr = sk->sk_v6_rcv_saddr;
+	addr->v6.sin6_scope_id = 0;
 }
 
 /* Initialize sk->sk_rcv_saddr from sctp_addr. */