[syzbot] [usb?] general protection fault in mct_u232_open

[syzbot <syzbot+58b2d8b19cd6761a395b@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    ddfe51b0a0f5 Add linux-next specific files for 20261105
git tree:       linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=12627abe580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3dffa811bfeed6b4
dashboard link: https://syzkaller.appspot.com/bug?extid=58b2d8b19cd6761a395b
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14f5f2be580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16627abe580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/181043a4bd52/disk-ddfe51b0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a69d8b670c38/vmlinux-ddfe51b0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/906d515c0893/bzImage-ddfe51b0.xz

AI-generated root cause summary:
In mct_u232_open(), if submitting the read URB via
usb_submit_urb(priv->read_urb, GFP_KERNEL) fails, the driver logs an error
message referencing port->read_urb->pipe instead of priv->read_urb->pipe.
Because the MCT U232 adapter uses interrupt-in endpoints instead of bulk-in
endpoints, port->read_urb is never allocated or set up by the usb-serial core
and remains NULL. Dereferencing port->read_urb->pipe in the error path results
in a NULL-pointer dereference at offset 0x50 (the offset of the pipe field in
struct urb), crashing the kernel with an oops.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=58b2d8b19cd6761a395b

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+58b2d8b19cd6761a395b@syzkaller.appspotmail.com

mct_u232 ttyUSB0: Set MODEM CTRL 0xb failed (error = -71)
mct_u232 ttyUSB0: Set LINE CTRL 0x3 failed (error = -71)
mct_u232 ttyUSB0: Get MODEM STATus failed (error = -19)
Oops: general protection fault, probably for non-canonical address 0xdffffc000000000a: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000050-0x0000000000000057]
CPU: 0 UID: 0 PID: 5718 Comm: syz-executor711 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
RIP: 0010:mct_u232_open+0x39c/0x680 drivers/usb/serial/mct_u232.c:460
Code: 8d 9e 20 04 00 00 48 89 d8 48 c1 e8 03 42 80 3c 38 00 74 08 48 89 df e8 62 c3 6e fa 48 8b 1b 48 83 c3 50 48 89 d8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 a7 01 00 00 8b 13 49 81 c6 40 05 00 00
RSP: 0018:ffffc90003b7f760 EFLAGS: 00010206
RAX: 000000000000000a RBX: 0000000000000050 RCX: ffff88802e889f40
RDX: 0000000000000000 RSI: 00000000ffffffed RDI: 0000000000000000
RBP: 00000000ffffffed R08: ffffffff91e120bf R09: 1ffffffff23c2417
R10: dffffc0000000000 R11: fffffbfff23c2418 R12: 1ffff11005c8bf50
R13: dffffc0000000000 R14: ffff888034df4000 R15: dffffc0000000000
FS:  0000555579e9e400(0000) GS:ffff888123159000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f2cf0c571bc CR3: 0000000037ff2000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 serial_port_activate+0x151/0x250 drivers/usb/serial/usb-serial.c:268
 tty_port_open+0x1df/0x2d0 drivers/tty/tty_port.c:798
 tty_open+0x48a/0xd80 drivers/tty/tty_io.c:2112
 chrdev_open+0x4d9/0x600 fs/char_dev.c:413
 do_dentry_open+0x80e/0x1360 fs/open.c:1000
 vfs_open_consume+0x4a/0x350 fs/open.c:1131
 do_open fs/namei.c:4851 [inline]
 path_openat+0x16fc/0x20f0 fs/namei.c:5016
 do_file_open+0x23e/0x4a0 fs/namei.c:5045
 do_sys_openat2+0x115/0x200 fs/open.c:1443
 do_sys_open fs/open.c:1449 [inline]
 __do_sys_openat fs/open.c:1789 [inline]
 __se_sys_openat fs/open.c:1784 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1784
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x16a/0x530 arch/x86/entry/syscall_64.c:89
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x00007f2cf0bf0079
Code: c0 79 93 eb d5 48 8d 7c 1d 00 eb 99 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 d8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe3866ad58 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2cf0bf0079
RDX: 0000000000000000 RSI: 0000200000000200 RDI: 00000000ffffff9c
RBP: 00000000000f4240 R08: 0000000000000009 R09: 0000000000000009
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000014f37
R13: 00007ffe3866ad6c R14: 00007ffe3866ad80 R15: 00007ffe3866ad70
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:mct_u232_open+0x39c/0x680 drivers/usb/serial/mct_u232.c:460
Code: 8d 9e 20 04 00 00 48 89 d8 48 c1 e8 03 42 80 3c 38 00 74 08 48 89 df e8 62 c3 6e fa 48 8b 1b 48 83 c3 50 48 89 d8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 a7 01 00 00 8b 13 49 81 c6 40 05 00 00
RSP: 0018:ffffc90003b7f760 EFLAGS: 00010206
RAX: 000000000000000a RBX: 0000000000000050 RCX: ffff88802e889f40
RDX: 0000000000000000 RSI: 00000000ffffffed RDI: 0000000000000000
RBP: 00000000ffffffed R08: ffffffff91e120bf R09: 1ffffffff23c2417
R10: dffffc0000000000 R11: fffffbfff23c2418 R12: 1ffff11005c8bf50
R13: dffffc0000000000 R14: ffff888034df4000 R15: dffffc0000000000
FS:  0000555579e9e400(0000) GS:ffff888123159000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f2cf0c92680 CR3: 0000000037ff2000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	8d 9e 20 04 00 00    	lea    0x420(%rsi),%ebx
   6:	48 89 d8             	mov    %rbx,%rax
   9:	48 c1 e8 03          	shr    $0x3,%rax
   d:	42 80 3c 38 00       	cmpb   $0x0,(%rax,%r15,1)
  12:	74 08                	je     0x1c
  14:	48 89 df             	mov    %rbx,%rdi
  17:	e8 62 c3 6e fa       	call   0xfa6ec37e
  1c:	48 8b 1b             	mov    (%rbx),%rbx
  1f:	48 83 c3 50          	add    $0x50,%rbx
  23:	48 89 d8             	mov    %rbx,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	42 0f b6 04 38       	movzbl (%rax,%r15,1),%eax <-- trapping instruction
  2f:	84 c0                	test   %al,%al
  31:	0f 85 a7 01 00 00    	jne    0x1de
  37:	8b 13                	mov    (%rbx),%edx
  39:	49 81 c6 40 05 00 00 	add    $0x540,%r14


---
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