Re: [syzbot] BUG: unable to handle kernel NULL pointer dereference in gsmld_receive_buf

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    ca688bff68bc Add linux-next specific files for 20220808
git tree:       linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=14a27066080000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4c20e006003cdecb
dashboard link: https://syzkaller.appspot.com/bug?extid=e3563f0c94e188366dbb
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=164eb3ca080000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17327ce1080000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e3563f0c94e188366dbb@xxxxxxxxxxxxxxxxxxxxxxxxx

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 74c90067 P4D 74c90067 PUD 74c53067 PMD 0 
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 3606 Comm: syz-executor237 Not tainted 5.19.0-next-20220808-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/22/2022
RIP: 0010:0x0
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
RSP: 0018:ffffc9000387fcf0 EFLAGS: 00010202
RAX: 0000000000000001 RBX: ffff88801bbdf000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88801bbdf000
RBP: ffffc9000387fda1 R08: 0000000000000001 R09: 0000000000000004
R10: 0000000000000000 R11: 1ffffffff1ffa9ce R12: 0000000000000001
R13: 0000000000000000 R14: ffffc9000387fd90 R15: dffffc0000000000
FS:  0000555556e8d300(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000001c7e0000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 gsmld_receive_buf+0x1c2/0x2f0 drivers/tty/n_gsm.c:2861
 tiocsti drivers/tty/tty_io.c:2293 [inline]
 tty_ioctl+0xa75/0x15d0 drivers/tty/tty_io.c:2692
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl fs/ioctl.c:856 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f76c0e5eb59
Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 00 00 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 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe9a4f7c48 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f76c0e5eb59
RDX: 0000000020000040 RSI: 0000000000005412 RDI: 0000000000000004
RBP: 00007f76c0e22d00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f76c0e22d90
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 </TASK>
Modules linked in:
CR2: 0000000000000000
---[ end trace 0000000000000000 ]---
RIP: 0010:0x0
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
RSP: 0018:ffffc9000387fcf0 EFLAGS: 00010202
RAX: 0000000000000001 RBX: ffff88801bbdf000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88801bbdf000
RBP: ffffc9000387fda1 R08: 0000000000000001 R09: 0000000000000004
R10: 0000000000000000 R11: 1ffffffff1ffa9ce R12: 0000000000000001
R13: 0000000000000000 R14: ffffc9000387fd90 R15: dffffc0000000000
FS:  0000555556e8d300(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000001c7e0000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400