netns: Make get_net_ns() handle zero refcount net

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

netns: Make get_net_ns() handle zero refcount net

Syzkaller hit a warning:
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 3 PID: 7890 at lib/refcount.c:25 refcount_warn_saturate+0xdf/0x1d0
Modules linked in:
CPU: 3 PID: 7890 Comm: tun Not tainted 6.10.0-rc3-00100-gcaa4f9578aba-dirty #310
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:refcount_warn_saturate+0xdf/0x1d0
Code: 41 49 04 31 ff 89 de e8 9f 1e cd fe 84 db 75 9c e8 76 26 cd fe c6 05 b6 41 49 04 01 90 48 c7 c7 b8 8e 25 86 e8 d2 05 b5 fe 90 <0f> 0b 90 90 e9 79 ff ff ff e8 53 26 cd fe 0f b6 1
RSP: 0018:ffff8881067b7da0 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff811c72ac
RDX: ffff8881026a2140 RSI: ffffffff811c72b5 RDI: 0000000000000001
RBP: ffff8881067b7db0 R08: 0000000000000000 R09: 205b5d3730353139
R10: 0000000000000000 R11: 205d303938375420 R12: ffff8881086500c4
R13: ffff8881086500c4 R14: ffff8881086500b0 R15: ffff888108650040
FS: 00007f5b2961a4c0(0000) GS:ffff88823bd00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055d7ed36fd18 CR3: 00000001482f6000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
? show_regs+0xa3/0xc0
? __warn+0xa5/0x1c0
? refcount_warn_saturate+0xdf/0x1d0
? report_bug+0x1fc/0x2d0
? refcount_warn_saturate+0xdf/0x1d0
? handle_bug+0xa1/0x110
? exc_invalid_op+0x3c/0xb0
? asm_exc_invalid_op+0x1f/0x30
? __warn_printk+0xcc/0x140
? __warn_printk+0xd5/0x140
? refcount_warn_saturate+0xdf/0x1d0
get_net_ns+0xa4/0xc0
? __pfx_get_net_ns+0x10/0x10
open_related_ns+0x5a/0x130
__tun_chr_ioctl+0x1616/0x2370
? __sanitizer_cov_trace_switch+0x58/0xa0
? __sanitizer_cov_trace_const_cmp2+0x1c/0x30
? __pfx_tun_chr_ioctl+0x10/0x10
tun_chr_ioctl+0x2f/0x40
__x64_sys_ioctl+0x11b/0x160
x64_sys_call+0x1211/0x20d0
do_syscall_64+0x9e/0x1d0
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f5b28f165d7
Code: b3 66 90 48 8b 05 b1 48 2d 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 81 48 2d 00 8
RSP: 002b:00007ffc2b59c5e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5b28f165d7
RDX: 0000000000000000 RSI: 00000000000054e3 RDI: 0000000000000003
RBP: 00007ffc2b59c650 R08: 00007f5b291ed8c0 R09: 00007f5b2961a4c0
R10: 0000000029690010 R11: 0000000000000246 R12: 0000000000400730
R13: 00007ffc2b59cf40 R14: 0000000000000000 R15: 0000000000000000
</TASK>
Kernel panic - not syncing: kernel: panic_on_warn set ...

This is trigger as below:
ns0 ns1
tun_set_iff() //dev is tun0
tun->dev = dev
//ip link set tun0 netns ns1
put_net() //ref is 0
__tun_chr_ioctl() //TUNGETDEVNETNS
net = dev_net(tun->dev);
open_related_ns(&net->ns, get_net_ns); //ns1
get_net_ns()
get_net() //addition on 0

Use maybe_get_net() in get_net_ns in case net's ref is zero to fix this