net: bridge: mst: prevent NULL deref in br_mst_info_size()

Call br_mst_info_size() only if vg pointer is not NULL.

general protection fault, probably for non-canonical address 0xdffffc0000000058: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000002c0-0x00000000000002c7]
CPU: 0 PID: 975 Comm: syz-executor.0 Tainted: G        W         5.17.0-next-20220321-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:br_mst_info_size+0x97/0x270 net/bridge/br_mst.c:242
Code: 00 00 31 c0 e8 ba 10 53 f9 31 c0 b9 40 00 00 00 4c 8d 6c 24 30 4c 89 ef f3 48 ab 48 8d 83 c0 02 00 00 48 89 04 24 48 c1 e8 03 <80> 3c 28 00 0f 85 ae 01 00 00 48 8b 83 c0 02 00 00 41 bf 04 00 00
RSP: 0018:ffffc900153770a8 EFLAGS: 00010202
RAX: 0000000000000058 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000040000 RSI: ffffffff88259876 RDI: ffffc900153772d8
RBP: dffffc0000000000 R08: 0000000000000000 R09: ffffffff8db68957
R10: ffffffff881f737b R11: 0000000000000000 R12: 0000000000000000
R13: ffffc900153770d8 R14: 00000000000002a0 R15: 00000000ffffffff
FS:  00007f18bbb6f700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020001a80 CR3: 000000001a7d9000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 00000000000000d8 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 br_get_link_af_size_filtered+0x6e9/0xc00 net/bridge/br_netlink.c:123
 rtnl_link_get_af_size net/core/rtnetlink.c:598 [inline]
 if_nlmsg_size+0x40c/0xa50 net/core/rtnetlink.c:1040
 rtnl_calcit.isra.0+0x25f/0x460 net/core/rtnetlink.c:3780
 rtnetlink_rcv_msg+0xa65/0xb80 net/core/rtnetlink.c:5937
 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2496
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x543/0x7f0 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x904/0xe00 net/netlink/af_netlink.c:1921
 sock_sendmsg_nosec net/socket.c:705 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:725
 ____sys_sendmsg+0x6e8/0x810 net/socket.c:2413
 ___sys_sendmsg+0xf3/0x170 net/socket.c:2467
 __sys_sendmsg+0xe5/0x1b0 net/socket.c:2496
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0x80 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f18baa89049
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f18bbb6f168 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f18bab9bf60 RCX: 00007f18baa89049
RDX: 0000000000000000 RSI: 0000000020001a80 RDI: 0000000000000004
RBP: 00007f18baae308d R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffeedb2be2f R14: 00007f18bbb6f300 R15: 0000000000022000
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:br_mst_info_size+0x97/0x270 net/bridge/br_mst.c:242
Code: 00 00 31 c0 e8 ba 10 53 f9 31 c0 b9 40 00 00 00 4c 8d 6c 24 30 4c 89 ef f3 48 ab 48 8d 83 c0 02 00 00 48 89 04 24 48 c1 e8 03 <80> 3c 28 00 0f 85 ae 01 00 00 48 8b 83 c0 02 00 00 41 bf 04 00 00
RSP: 0018:ffffc900153770a8 EFLAGS: 00010202
RAX: 0000000000000058 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000040000 RSI: ffffffff88259876 RDI: ffffc900153772d8
RBP: dffffc0000000000 R08: 0000000000000000 R09: ffffffff8db68957
R10: ffffffff881f737b R11: 0000000000000000 R12: 0000000000000000
R13: ffffc900153770d8 R14: 00000000000002a0 R15: 00000000ffffffff
FS:  00007f18bbb6f700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2ca22000 CR3: 000000001a7d9000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 00000000000000d8 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Fixes: 122c2948 ("net: bridge: mst: Support setting and reporting MST port states")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tobias Waldekranz <tobias@waldekranz.com>
Cc: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20220322012314.795187-1-eric.dumazet@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>