BUG: unable to handle kernel paging request in slhc_free

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    8fe28cb58bcb Linux 4.20
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16771b15400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7d581260bae0899a
dashboard link: https://syzkaller.appspot.com/bug?extid=6c5d567447bfa30f78e2
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=136130fd400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1607c563400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+6c5d567447bfa30f78e2@xxxxxxxxxxxxxxxxxxxxxxxxx

RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440359
RDX: 0000000020000080 RSI: 0000000000005423 RDI: 0000000000000003
RBP: 00000000006cb018 R08: 0000000000000001 R09: 0000000000000039
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000004
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
BUG: unable to handle kernel paging request at fffffffffffffff4
PGD 946d067 P4D 946d067 PUD 946f067 PMD 0
Oops: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 8225 Comm: syz-executor096 Not tainted 4.20.0 #167
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:slhc_free+0x30/0xb0 drivers/net/slip/slhc.c:159
Code: 53 48 89 fb e8 71 04 ad fc 48 85 db 74 74 e8 67 04 ad fc 48 89 da 48  
b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 69 <4c> 8b 23 4d 85  
e4 74 0d e8 43 04 ad fc 4c 89 e7 e8 bb e1 ef fc e8
RSP: 0018:ffff8881c1a0f770 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: fffffffffffffff4 RCX: 1ffff1103408b936
RDX: 1ffffffffffffffe RSI: ffffffff84d15439 RDI: fffffffffffffff4
RBP: ffff8881c1a0f780 R08: ffff8881a045c9b0 R09: 0000000000000006
R10: 0000000000000000 R11: ffff8881a045c0c0 R12: 00000000ffffff97
R13: ffff8881c0ca04c0 R14: ffff8881c0daed00 R15: ffff8881c2152ac0
FS:  000000000078a880(0000) GS:ffff8881daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: fffffffffffffff4 CR3: 00000001b0909000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 sl_alloc_bufs drivers/net/slip/slip.c:196 [inline]
 slip_open+0xdee/0x1107 drivers/net/slip/slip.c:821
 tty_ldisc_open.isra.1+0x8b/0xe0 drivers/tty/tty_ldisc.c:453
 tty_set_ldisc+0x2dc/0x6a0 drivers/tty/tty_ldisc.c:578
 tiocsetd drivers/tty/tty_io.c:2321 [inline]
 tty_ioctl+0x39d/0x17d0 drivers/tty/tty_io.c:2581
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:509 [inline]
 do_vfs_ioctl+0x1de/0x1790 fs/ioctl.c:696
 ksys_ioctl+0xa9/0xd0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440359
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 0f 83 5b 14 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff63454bb8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440359
RDX: 0000000020000080 RSI: 0000000000005423 RDI: 0000000000000003
RBP: 00000000006cb018 R08: 0000000000000001 R09: 0000000000000039
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000004
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
CR2: fffffffffffffff4
---[ end trace 45b654871fafdc18 ]---
RIP: 0010:slhc_free+0x30/0xb0 drivers/net/slip/slhc.c:159
Code: 53 48 89 fb e8 71 04 ad fc 48 85 db 74 74 e8 67 04 ad fc 48 89 da 48  
b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 69 <4c> 8b 23 4d 85  
e4 74 0d e8 43 04 ad fc 4c 89 e7 e8 bb e1 ef fc e8
RSP: 0018:ffff8881c1a0f770 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: fffffffffffffff4 RCX: 1ffff1103408b936
RDX: 1ffffffffffffffe RSI: ffffffff84d15439 RDI: fffffffffffffff4
RBP: ffff8881c1a0f780 R08: ffff8881a045c9b0 R09: 0000000000000006
R10: 0000000000000000 R11: ffff8881a045c0c0 R12: 00000000ffffff97
R13: ffff8881c0ca04c0 R14: ffff8881c0daed00 R15: ffff8881c2152ac0
FS:  000000000078a880(0000) GS:ffff8881daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: fffffffffffffff4 CR3: 00000001b0909000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
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