syzbot was able to trigger a crash [1] in page_pool_unlist()

page_pool_list() only inserts a page pool into a netdev page pool list
if a netdev was set in params.

Even if the kzalloc() call in page_pool_create happens to initialize
pool->user.list, I chose to be more explicit in page_pool_list()
adding one INIT_HLIST_NODE().

We could test in page_pool_unlist() if netdev was set,
but since netdev can be changed to lo, it seems more robust to
check if pool->user.list is hashed  before calling hlist_del().

[1]

Illegal XDP return value 4294946546 on prog  (id 2) dev N/A, expect packet loss!
general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 PID: 5064 Comm: syz-executor391 Not tainted 6.7.0-rc2-syzkaller-00533-ga3799729 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/10/2023
RIP: 0010:__hlist_del include/linux/list.h:988 [inline]
RIP: 0010:hlist_del include/linux/list.h:1002 [inline]
RIP: 0010:page_pool_unlist+0xd1/0x170 net/core/page_pool_user.c:342
Code: df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 90 00 00 00 4c 8b a3 f0 06 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 75 68 48 85 ed 49 89 2c 24 74 24 e8 1b ca 07 f9 48 8d
RSP: 0018:ffffc900039ff768 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff88814ae02000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffff88814ae026f0
RBP: 0000000000000000 R08: 0000000000000000 R09: fffffbfff1d57fdc
R10: ffffffff8eabfee3 R11: ffffffff8aa0008b R12: 0000000000000000
R13: ffff88814ae02000 R14: dffffc0000000000 R15: 0000000000000001
FS:  000055555717a380(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000002555398 CR3: 0000000025044000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __page_pool_destroy net/core/page_pool.c:851 [inline]
 page_pool_release+0x507/0x6b0 net/core/page_pool.c:891
 page_pool_destroy+0x1ac/0x4c0 net/core/page_pool.c:956
 xdp_test_run_teardown net/bpf/test_run.c:216 [inline]
 bpf_test_run_xdp_live+0x1578/0x1af0 net/bpf/test_run.c:388
 bpf_prog_test_run_xdp+0x827/0x1530 net/bpf/test_run.c:1254
 bpf_prog_test_run kernel/bpf/syscall.c:4041 [inline]
 __sys_bpf+0x11bf/0x4920 kernel/bpf/syscall.c:5402
 __do_sys_bpf kernel/bpf/syscall.c:5488 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:5486 [inline]
 __x64_sys_bpf+0x78/0xc0 kernel/bpf/syscall.c:5486

Fixes: 083772c9 ("net: page_pool: record pools per netdev")
Reported-and-tested-by: syzbot+f9f8efb58a4db2ca98d0@syzkaller.appspotmail.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231130092259.3797753-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>