[BUG] null-ptr-deref bug in sctp_packet_bundle_auth

Hi all,

We tested with syzkaller on linux-6.6 and it reported a null-ptr-deref bug:

==================================================================
general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 0 PID: 16 Comm: ksoftirqd/0 Tainted: G W 6.6.0 #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
RIP: 0010:sctp_packet_bundle_auth net/sctp/output.c:264 [inline]
RIP: 0010:sctp_packet_append_chunk+0xb36/0x1260 net/sctp/output.c:401
Code: 4c 89 f2 48 c1 ea 03 80 3c 02 00 0f 85 93 05 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b 7b 40 49 8d 7f 1c 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 57
RSP: 0018:ffffc9000010efe8 EFLAGS: 00010207
RAX: dffffc0000000000 RBX: ffff88810c746000 RCX: ffffffff8d758fca
RDX: 0000000000000003 RSI: ffff888100d1cd40 RDI: 000000000000001c
RBP: ffff88811dea8b78 R08: 0000000000000000 R09: fffffbfff27f8b7d
R10: fffffbfff27f8b7c R11: ffffffff93fc5be7 R12: ffff88804247c000
R13: ffff88811dea8bb8 R14: ffff88810c746040 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88816e600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000021000000 CR3: 000000011dfb4002 CR4: 0000000000170ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:

sctp_packet_transmit_chunk+0x31/0x250 net/sctp/output.c:189
sctp_outq_flush_data+0xa29/0x26d0 net/sctp/outqueue.c:1111
sctp_outq_flush+0xc80/0x1240 net/sctp/outqueue.c:1217
sctp_cmd_interpreter.isra.0+0x19a5/0x62c0 net/sctp/sm_sideeffect.c:1787
sctp_side_effects net/sctp/sm_sideeffect.c:1198 [inline]
sctp_do_sm+0x1a3/0x670 net/sctp/sm_sideeffect.c:1169
sctp_assoc_bh_rcv+0x33e/0x640 net/sctp/associola.c:1052
sctp_inq_push+0x1dd/0x280 net/sctp/inqueue.c:88
sctp_rcv+0x11ae/0x3100 net/sctp/input.c:243
sctp6_rcv+0x3d/0x60 net/sctp/ipv6.c:1127
ip6_protocol_deliver_rcu+0x12c1/0x1e40 net/ipv6/ip6_input.c:438
ip6_input_finish+0xc8/0x260 net/ipv6/ip6_input.c:483
NF_HOOK include/linux/netfilter.h:308 [inline]
ip6_input+0xd1/0x430 net/ipv6/ip6_input.c:492
dst_input include/net/dst.h:487 [inline]
ip6_rcv_finish+0x1b9/0x5c0 net/ipv6/ip6_input.c:79
NF_HOOK include/linux/netfilter.h:308 [inline]
ipv6_rcv+0xf8/0x440 net/ipv6/ip6_input.c:310
__netif_receive_skb_one_core+0x133/0x1f0 net/core/dev.c:5603
__netif_receive_skb+0x24/0x1c0 net/core/dev.c:5717
process_backlog+0x21d/0x7e0 net/core/dev.c:6045
__napi_poll+0xc2/0x690 net/core/dev.c:6607
napi_poll net/core/dev.c:6674 [inline]
net_rx_action+0x87d/0xce0 net/core/dev.c:6810
handle_softirqs+0x1b4/0x700 kernel/softirq.c:578
run_ksoftirqd kernel/softirq.c:997 [inline]
run_ksoftirqd+0x2f/0x60 kernel/softirq.c:989
smpboot_thread_fn+0x3b9/0x860 kernel/smpboot.c:164
kthread+0x324/0x420 kernel/kthread.c:388
ret_from_fork+0x49/0x80 arch/x86/kernel/process.c:152
ret_from_fork_asm+0x1b/0x30 arch/x86/entry/entry_64.S:293

Modules linked in:
kernel fault(0x1) notification starting on CPU 0
kernel fault(0x1) notification finished on CPU 0
---[ end trace 0000000000000000 ]---
RIP: 0010:sctp_packet_bundle_auth net/sctp/output.c:264 [inline]
RIP: 0010:sctp_packet_append_chunk+0xb36/0x1260 net/sctp/output.c:401
Code: 4c 89 f2 48 c1 ea 03 80 3c 02 00 0f 85 93 05 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b 7b 40 49 8d 7f 1c 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 57
RSP: 0018:ffffc9000010efe8 EFLAGS: 00010207
RAX: dffffc0000000000 RBX: ffff88810c746000 RCX: ffffffff8d758fca
RDX: 0000000000000003 RSI: ffff888100d1cd40 RDI: 000000000000001c
RBP: ffff88811dea8b78 R08: 0000000000000000 R09: fffffbfff27f8b7d
R10: fffffbfff27f8b7c R11: ffffffff93fc5be7 R12: ffff88804247c000
R13: ffff88811dea8bb8 R14: ffff88810c746040 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88816e600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000021000000 CR3: 000000011dfb4002 CR4: 0000000000170ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Code disassembly (best guess):
0: 4c 89 f2 mov %r14,%rdx
3: 48 c1 ea 03 shr $0x3,%rdx
7: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1)
b: 0f 85 93 05 00 00 jne 0x5a4
11: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
18: fc ff df
1b: 4c 8b 7b 40 mov 0x40(%rbx),%r15
1f: 49 8d 7f 1c lea 0x1c(%r15),%rdi
23: 48 89 fa mov %rdi,%rdx
26: 48 c1 ea 03 shr $0x3,%rdx

2a: 0f b6 14 02 movzbl (%rdx,%rax,1),%edx <-- trapping instruction
2e: 48 89 f8 mov %rdi,%rax
31: 83 e0 07 and $0x7,%eax
34: 83 c0 01 add $0x1,%eax
37: 38 d0 cmp %dl,%al
39: 7c 08 jl 0x43
3b: 84 d2 test %dl,%dl
3d: 0f .byte 0xf
3e: 85 .byte 0x85
3f: 57 push %rdi
==================================================================

The direct cause is that chunk->shkey is NULL in sctp_packet_bundle_auth().

	if (!chunk->auth)
		return retval;

	auth = sctp_make_auth(asoc, chunk->shkey->key_id);    <=== BUG
	if (!auth)
		return retval;

It is simple to fix it by adding a null-ptr check before sctp_make_auth() but maybe
there are better solutions?

Thanks.