I observed this when unplugging a DP monitor whilst a computer is asleep
and then waking it up. This left DP chardev nodes still being present on
the filesystem and accessing these device nodes caused an oops because
drm_dp_aux_dev_get_by_minor() assumes a device exists if it is opened.
This can also be reproduced by creating a device node with mknod(1) and
issuing an open(2)

[166164.933198] BUG: kernel NULL pointer dereference, address: 0000000000000018
[166164.933202] #PF: supervisor read access in kernel mode
[166164.933204] #PF: error_code(0x0000) - not-present page
[166164.933205] PGD 0 P4D 0
[166164.933208] Oops: 0000 [#1] PREEMPT SMP NOPTI
[166164.933211] CPU: 4 PID: 99071 Comm: fwupd Tainted: G        W
5.8.0-rc6+ #1
[166164.933213] Hardware name: LENOVO 20RD002VUS/20RD002VUS, BIOS R16ET25W
(1.11 ) 04/21/2020
[166164.933232] RIP: 0010:drm_dp_aux_dev_get_by_minor+0x29/0x70
[drm_kms_helper]
[166164.933234] Code: 00 0f 1f 44 00 00 55 48 89 e5 41 54 41 89 fc 48 c7
c7 60 01 a4 c0 e8 26 ab 30 d7 44 89 e6 48 c7 c7 80 01 a4 c0 e8 47 94 d6 d6
<8b> 50 18 49 89 c4 48 8d 78 18 85 d2 74 33 8d 4a 01 89 d0 f0 0f b1
[166164.933236] RSP: 0018:ffffb7d7c41cbbf0 EFLAGS: 00010246
[166164.933237] RAX: 0000000000000000 RBX: ffff8a90001fe900 RCX: 0000000000000000
[166164.933238] RDX: 0000000000000000 RSI: 0000000000000003 RDI: ffffffffc0a40180
[166164.933239] RBP: ffffb7d7c41cbbf8 R08: 0000000000000000 R09: ffff8a93e157d6d0
[166164.933240] R10: 0000000000000000 R11: ffffffffc0a40188 R12: 0000000000000003
[166164.933241] R13: ffff8a9402200e80 R14: ffff8a90001fe900 R15: 0000000000000000
[166164.933244] FS:  00007f7fb041eb00(0000) GS:ffff8a9411500000(0000)
knlGS:0000000000000000
[166164.933245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[166164.933246] CR2: 0000000000000018 CR3: 00000000352c2003 CR4: 00000000003606e0
[166164.933247] Call Trace:
[166164.933264]  auxdev_open+0x1b/0x40 [drm_kms_helper]
[166164.933278]  chrdev_open+0xa7/0x1c0
[166164.933282]  ? cdev_put.part.0+0x20/0x20
[166164.933287]  do_dentry_open+0x161/0x3c0
[166164.933291]  vfs_open+0x2d/0x30
[166164.933297]  path_openat+0xb27/0x10e0
[166164.933306]  ? atime_needs_update+0x73/0xd0
[166164.933309]  do_filp_open+0x91/0x100
[166164.933313]  ? __alloc_fd+0xb2/0x150
[166164.933316]  do_sys_openat2+0x210/0x2d0
[166164.933318]  do_sys_open+0x46/0x80
[166164.933320]  __x64_sys_openat+0x20/0x30
[166164.933328]  do_syscall_64+0x52/0xc0
[166164.933336]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

(gdb) disassemble drm_dp_aux_dev_get_by_minor+0x29
Dump of assembler code for function drm_dp_aux_dev_get_by_minor:
   0x0000000000017b10 <+0>:     callq  0x17b15 <drm_dp_aux_dev_get_by_minor+5>
   0x0000000000017b15 <+5>:     push   %rbp
   0x0000000000017b16 <+6>:     mov    %rsp,%rbp
   0x0000000000017b19 <+9>:     push   %r12
   0x0000000000017b1b <+11>:    mov    %edi,%r12d
   0x0000000000017b1e <+14>:    mov    $0x0,%rdi
   0x0000000000017b25 <+21>:    callq  0x17b2a <drm_dp_aux_dev_get_by_minor+26>
   0x0000000000017b2a <+26>:    mov    %r12d,%esi
   0x0000000000017b2d <+29>:    mov    $0x0,%rdi
   0x0000000000017b34 <+36>:    callq  0x17b39 <drm_dp_aux_dev_get_by_minor+41>
   0x0000000000017b39 <+41>:    mov    0x18(%rax),%edx <=========
   0x0000000000017b3c <+44>:    mov    %rax,%r12
   0x0000000000017b3f <+47>:    lea    0x18(%rax),%rdi
   0x0000000000017b43 <+51>:    test   %edx,%edx
   0x0000000000017b45 <+53>:    je     0x17b7a <drm_dp_aux_dev_get_by_minor+106>
   0x0000000000017b47 <+55>:    lea    0x1(%rdx),%ecx
   0x0000000000017b4a <+58>:    mov    %edx,%eax
   0x0000000000017b4c <+60>:    lock cmpxchg %ecx,(%rdi)
   0x0000000000017b50 <+64>:    jne    0x17b76 <drm_dp_aux_dev_get_by_minor+102>
   0x0000000000017b52 <+66>:    test   %edx,%edx
   0x0000000000017b54 <+68>:    js     0x17b6d <drm_dp_aux_dev_get_by_minor+93>
   0x0000000000017b56 <+70>:    test   %ecx,%ecx
   0x0000000000017b58 <+72>:    js     0x17b6d <drm_dp_aux_dev_get_by_minor+93>
   0x0000000000017b5a <+74>:    mov    $0x0,%rdi
   0x0000000000017b61 <+81>:    callq  0x17b66 <drm_dp_aux_dev_get_by_minor+86>
   0x0000000000017b66 <+86>:    mov    %r12,%rax
   0x0000000000017b69 <+89>:    pop    %r12
   0x0000000000017b6b <+91>:    pop    %rbp
   0x0000000000017b6c <+92>:    retq
   0x0000000000017b6d <+93>:    xor    %esi,%esi
   0x0000000000017b6f <+95>:    callq  0x17b74 <drm_dp_aux_dev_get_by_minor+100>
   0x0000000000017b74 <+100>:   jmp    0x17b5a <drm_dp_aux_dev_get_by_minor+74>
   0x0000000000017b76 <+102>:   mov    %eax,%edx
   0x0000000000017b78 <+104>:   jmp    0x17b43 <drm_dp_aux_dev_get_by_minor+51>
   0x0000000000017b7a <+106>:   xor    %r12d,%r12d
   0x0000000000017b7d <+109>:   jmp    0x17b5a <drm_dp_aux_dev_get_by_minor+74>
End of assembler dump.

(gdb) list *drm_dp_aux_dev_get_by_minor+0x29
0x17b39 is in drm_dp_aux_dev_get_by_minor (drivers/gpu/drm/drm_dp_aux_dev.c:65).
60      static struct drm_dp_aux_dev *drm_dp_aux_dev_get_by_minor(unsigned index)
61      {
62              struct drm_dp_aux_dev *aux_dev = NULL;
63
64              mutex_lock(&aux_idr_mutex);
65              aux_dev = idr_find(&aux_idr, index);
66              if (!kref_get_unless_zero(&aux_dev->refcount))
67                      aux_dev = NULL;
68              mutex_unlock(&aux_idr_mutex);
69
(gdb) p/x &((struct drm_dp_aux_dev *)(0x0))->refcount
$8 = 0x18

Looking at the caller, checks on the minor are pushed down to
drm_dp_aux_dev_get_by_minor()

static int auxdev_open(struct inode *inode, struct file *file)
{
    unsigned int minor = iminor(inode);
    struct drm_dp_aux_dev *aux_dev;

    aux_dev = drm_dp_aux_dev_get_by_minor(minor); <====
    if (!aux_dev)
        return -ENODEV;

    file->private_data = aux_dev;
    return 0;
}

Fixes: e94cb37b ("drm/dp: Add a drm_aux-dev module for reading/writing dpcd registers.")
Cc: <stable@vger.kernel.org> # v4.6+
Signed-off-by: Zwane Mwaikambo <zwane@yosper.io>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[added Cc to stable]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/alpine.DEB.2.21.2010122231070.38717@montezuma.home