sctp: fix possible UAF in sctp_v6_available()

In the Linux kernel, the following vulnerability has been resolved:

sctp: fix possible UAF in sctp_v6_available()

A lockdep report [1] with CONFIG_PROVE_RCU_LIST=y hints
that sctp_v6_available() is calling dev_get_by_index_rcu()
and ipv6_chk_addr() without holding rcu.

[1]
=============================
WARNING: suspicious RCU usage
6.12.0-rc5-virtme #1216 Tainted: G W
-----------------------------
net/core/dev.c:876 RCU-list traversed in non-reader section!!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by sctp_hello/31495:
#0: ffff9f1ebbdb7418 (sk_lock-AF_INET6){+.+.}-{0:0}, at: sctp_bind (./arch/x86/include/asm/jump_label.h:27 net/sctp/socket.c:315) sctp

stack backtrace:
CPU: 7 UID: 0 PID: 31495 Comm: sctp_hello Tainted: G W 6.12.0-rc5-virtme #1216
Tainted: [W]=WARN
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl (lib/dump_stack.c:123)
lockdep_rcu_suspicious (kernel/locking/lockdep.c:6822)
dev_get_by_index_rcu (net/core/dev.c:876 (discriminator 7))
sctp_v6_available (net/sctp/ipv6.c:701) sctp
sctp_do_bind (net/sctp/socket.c:400 (discriminator 1)) sctp
sctp_bind (net/sctp/socket.c:320) sctp
inet6_bind_sk (net/ipv6/af_inet6.c:465)
? security_socket_bind (security/security.c:4581 (discriminator 1))
__sys_bind (net/socket.c:1848 net/socket.c:1869)
? do_user_addr_fault (./include/linux/rcupdate.h:347 ./include/linux/rcupdate.h:880 ./include/linux/mm.h:729 arch/x86/mm/fault.c:1340)
? do_user_addr_fault (./arch/x86/include/asm/preempt.h:84 (discriminator 13) ./include/linux/rcupdate.h:98 (discriminator 13) ./include/linux/rcupdate.h:882 (discriminator 13) ./include/linux/mm.h:729 (discriminator 13) arch/x86/mm/fault.c:1340 (discriminator 13))
__x64_sys_bind (net/socket.c:1877 (discriminator 1) net/socket.c:1875 (discriminator 1) net/socket.c:1875 (discriminator 1))
do_syscall_64 (arch/x86/entry/common.c:52 (discriminator 1) arch/x86/entry/common.c:83 (discriminator 1))
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f59b934a1e7
Code: 44 00 00 48 8b 15 39 8c 0c 00 f7 d8 64 89 02 b8 ff ff ff ff eb bd 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 b8 31 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 09 8c 0c 00 f7 d8 64 89 01 48
All code
========
0: 44 00 00 add %r8b,(%rax)
3: 48 8b 15 39 8c 0c 00 mov 0xc8c39(%rip),%rdx # 0xc8c43
a: f7 d8 neg %eax
c: 64 89 02 mov %eax,%fs:(%rdx)
f: b8 ff ff ff ff mov $0xffffffff,%eax
14: eb bd jmp 0xffffffffffffffd3
16: 66 2e 0f 1f 84 00 00 cs nopw 0x0(%rax,%rax,1)
1d: 00 00 00
20: 0f 1f 00 nopl (%rax)
23: b8 31 00 00 00 mov $0x31,%eax
28: 0f 05 syscall
2a:* 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax <-- trapping instruction
30: 73 01 jae 0x33
32: c3 ret
33: 48 8b 0d 09 8c 0c 00 mov 0xc8c09(%rip),%rcx # 0xc8c43
3a: f7 d8 neg %eax
3c: 64 89 01 mov %eax,%fs:(%rcx)
3f: 48 rex.W

Code starting with the faulting instruction
===========================================
0: 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax
6: 73 01 jae 0x9
8: c3 ret
9: 48 8b 0d 09 8c 0c 00 mov 0xc8c09(%rip),%rcx # 0xc8c19
10: f7 d8 neg %eax
12: 64 89 01 mov %eax,%fs:(%rcx)
15: 48 rex.W
RSP: 002b:00007ffe2d0ad398 EFLAGS: 00000202 ORIG_RAX: 0000000000000031
RAX: ffffffffffffffda RBX: 00007ffe2d0ad3d0 RCX: 00007f59b934a1e7
RDX: 000000000000001c RSI: 00007ffe2d0ad3d0 RDI: 0000000000000005
RBP: 0000000000000005 R08: 1999999999999999 R09: 0000000000000000
R10: 00007f59b9253298 R11: 000000000000
---truncated---