[syzbot ci] Re: TUN/TAP: Improving throughput and latency by avoiding SKB drops

[syzbot ci <syzbot+cieaa424e2117e71dd@syzkaller.appspotmail.com>]

syzbot ci has tested the following series

[v1] TUN/TAP: Improving throughput and latency by avoiding SKB drops
https://lore.kernel.org/all/20250808153721.261334-1-simon.schippers@tu-dortmund.de
* [PATCH net] TUN/TAP: Improving throughput and latency by avoiding SKB drops

and found the following issue:
general protection fault in tun_net_xmit

Full report is available here:
https://ci.syzbot.org/series/4a9dd6ad-3c81-4957-b447-4d1e8e9ee7a2

***

general protection fault in tun_net_xmit

tree:      net
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git
base:      ae633388cae349886f1a3cfb27aa092854b24c1b
arch:      amd64
compiler:  Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
config:    https://ci.syzbot.org/builds/f35af9e4-44af-4a13-8842-d9d36ecb06e7/config
C repro:   https://ci.syzbot.org/findings/e400bf02-40dc-43bb-8c15-d21b7ecb7304/c_repro
syz repro: https://ci.syzbot.org/findings/e400bf02-40dc-43bb-8c15-d21b7ecb7304/syz_repro

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
CPU: 1 UID: 0 PID: 12 Comm: kworker/u8:0 Not tainted 6.16.0-syzkaller-06620-gae633388cae3-dirty #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Workqueue: ipv6_addrconf addrconf_dad_work
RIP: 0010:__ptr_ring_full include/linux/ptr_ring.h:51 [inline]
RIP: 0010:ptr_ring_full include/linux/ptr_ring.h:59 [inline]
RIP: 0010:tun_net_xmit+0x3ee/0x19c0 drivers/net/tun.c:1026
Code: 54 24 18 48 89 d0 48 c1 e8 03 48 89 44 24 58 42 0f b6 04 28 84 c0 0f 85 f9 11 00 00 48 63 02 48 8d 1c c3 48 89 d8 48 c1 e8 03 <42> 80 3c 28 00 74 08 48 89 df e8 d3 0f ac fb 48 8b 1b 48 8b 7c 24
RSP: 0018:ffffc900000f6f00 EFLAGS: 00010202
RAX: 0000000000000002 RBX: 0000000000000010 RCX: dffffc0000000000
RDX: ffff88811bf90940 RSI: 0000000000000004 RDI: ffffc900000f6e80
RBP: ffffc900000f7050 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff5200001edd0 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff8881054c8000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8881a3c80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000002280 CR3: 0000000110b70000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 __netdev_start_xmit include/linux/netdevice.h:5219 [inline]
 netdev_start_xmit include/linux/netdevice.h:5228 [inline]
 xmit_one net/core/dev.c:3827 [inline]
 dev_hard_start_xmit+0x2d7/0x830 net/core/dev.c:3843
 sch_direct_xmit+0x241/0x4b0 net/sched/sch_generic.c:344
 __dev_xmit_skb net/core/dev.c:4102 [inline]
 __dev_queue_xmit+0x1857/0x3b50 net/core/dev.c:4679
 neigh_output include/net/neighbour.h:547 [inline]
 ip6_finish_output2+0x11fe/0x16a0 net/ipv6/ip6_output.c:141
 NF_HOOK include/linux/netfilter.h:318 [inline]
 ndisc_send_skb+0xb54/0x1440 net/ipv6/ndisc.c:512
 addrconf_dad_completed+0x7ae/0xd60 net/ipv6/addrconf.c:4360
 addrconf_dad_work+0xc36/0x14b0 net/ipv6/addrconf.c:-1
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3321
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3402
 kthread+0x711/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__ptr_ring_full include/linux/ptr_ring.h:51 [inline]
RIP: 0010:ptr_ring_full include/linux/ptr_ring.h:59 [inline]
RIP: 0010:tun_net_xmit+0x3ee/0x19c0 drivers/net/tun.c:1026
Code: 54 24 18 48 89 d0 48 c1 e8 03 48 89 44 24 58 42 0f b6 04 28 84 c0 0f 85 f9 11 00 00 48 63 02 48 8d 1c c3 48 89 d8 48 c1 e8 03 <42> 80 3c 28 00 74 08 48 89 df e8 d3 0f ac fb 48 8b 1b 48 8b 7c 24
RSP: 0018:ffffc900000f6f00 EFLAGS: 00010202
RAX: 0000000000000002 RBX: 0000000000000010 RCX: dffffc0000000000
RDX: ffff88811bf90940 RSI: 0000000000000004 RDI: ffffc900000f6e80
RBP: ffffc900000f7050 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff5200001edd0 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff8881054c8000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8881a3c80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000002280 CR3: 0000000110b70000 CR4: 00000000000006f0
----------------
Code disassembly (best guess):
   0:	54                   	push   %rsp
   1:	24 18                	and    $0x18,%al
   3:	48 89 d0             	mov    %rdx,%rax
   6:	48 c1 e8 03          	shr    $0x3,%rax
   a:	48 89 44 24 58       	mov    %rax,0x58(%rsp)
   f:	42 0f b6 04 28       	movzbl (%rax,%r13,1),%eax
  14:	84 c0                	test   %al,%al
  16:	0f 85 f9 11 00 00    	jne    0x1215
  1c:	48 63 02             	movslq (%rdx),%rax
  1f:	48 8d 1c c3          	lea    (%rbx,%rax,8),%rbx
  23:	48 89 d8             	mov    %rbx,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	42 80 3c 28 00       	cmpb   $0x0,(%rax,%r13,1) <-- trapping instruction
  2f:	74 08                	je     0x39
  31:	48 89 df             	mov    %rbx,%rdi
  34:	e8 d3 0f ac fb       	call   0xfbac100c
  39:	48 8b 1b             	mov    (%rbx),%rbx
  3c:	48                   	rex.W
  3d:	8b                   	.byte 0x8b
  3e:	7c 24                	jl     0x64


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syzbot@syzkaller.appspotmail.com

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