bisecting fixing commit since 106fa147d3daa58d2c1ae5f41a29d07036fe7d0a
building syzkaller on 7751efd04aebb07bc82b5c0e8eeaca07be1ae112
testing commit 106fa147d3daa58d2c1ae5f41a29d07036fe7d0a with gcc (GCC) 8.1.0
kernel signature: 3ad600b0bc7cb93a58068e08741834dff5e72f8ac2aa6d12d346bf4be926698d
all runs: crashed: KASAN: null-ptr-deref Read in drm_dp_aux_dev_get_by_minor
testing current HEAD 399849e4654ea496a6217ba4e5ee3d304c995ab4
testing commit 399849e4654ea496a6217ba4e5ee3d304c995ab4 with gcc (GCC) 8.1.0
kernel signature: 39dfd1fad51ef6bfdd4345160b0c5cb13aaf7c2840a6520235dc14914ef8573f
all runs: crashed: KASAN: null-ptr-deref Read in drm_dp_aux_dev_get_by_minor
revisions tested: 2, total time: 24m21.938579954s (build: 17m0.61947429s, test: 6m18.281022649s)
the crash still happens on HEAD
commit msg: Linux 4.19.131
crash: KASAN: null-ptr-deref Read in drm_dp_aux_dev_get_by_minor
batman_adv: It is strongly recommended to keep mac addresses unique to avoid problems!
batman_adv: The newly added mac address (aa:aa:aa:aa:aa:3e) already exists on: batadv_slave_1
MINIX-fs: mounting unchecked file system, running fsck is recommended
batman_adv: It is strongly recommended to keep mac addresses unique to avoid problems!
==================================================================
BUG: KASAN: null-ptr-deref in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
BUG: KASAN: null-ptr-deref in refcount_add_not_zero arch/x86/include/asm/refcount.h:85 [inline]
BUG: KASAN: null-ptr-deref in refcount_inc_not_zero arch/x86/include/asm/refcount.h:107 [inline]
BUG: KASAN: null-ptr-deref in kref_get_unless_zero include/linux/kref.h:116 [inline]
BUG: KASAN: null-ptr-deref in drm_dp_aux_dev_get_by_minor+0x98/0x1b0 drivers/gpu/drm/drm_dp_aux_dev.c:63
batman_adv: The newly added mac address (aa:aa:aa:aa:aa:3e) already exists on: batadv_slave_1
Read of size 4 at addr 0000000000000018 by task syz-executor.5/7857

batman_adv: It is strongly recommended to keep mac addresses unique to avoid problems!
CPU: 0 PID: 7857 Comm: syz-executor.5 Not tainted 4.19.131-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x17c/0x22a lib/dump_stack.c:118
 kasan_report_error mm/kasan/report.c:352 [inline]
 kasan_report.cold.7+0x6d/0x307 mm/kasan/report.c:412
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x13c/0x1b0 mm/kasan/kasan.c:267
IPv6: ADDRCONF(NETDEV_UP): batadv_slave_1: link is not ready
 kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
 atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
 refcount_add_not_zero arch/x86/include/asm/refcount.h:85 [inline]
 refcount_inc_not_zero arch/x86/include/asm/refcount.h:107 [inline]
 kref_get_unless_zero include/linux/kref.h:116 [inline]
 drm_dp_aux_dev_get_by_minor+0x98/0x1b0 drivers/gpu/drm/drm_dp_aux_dev.c:63
 auxdev_open+0x49/0xa0 drivers/gpu/drm/drm_dp_aux_dev.c:131
 chrdev_open+0x1ed/0x5c0 fs/char_dev.c:423
 do_dentry_open+0x3f1/0x1010 fs/open.c:796
 vfs_open+0x9a/0xc0 fs/open.c:902
 do_last fs/namei.c:3421 [inline]
 path_openat+0x905/0x2870 fs/namei.c:3537
batman_adv: batadv0: Interface activated: batadv_slave_1
 do_filp_open+0x177/0x250 fs/namei.c:3567
 file_open_name+0x13c/0x1a0 fs/open.c:1032
audit: type=1800 audit(1594197570.245:13): pid=7864 uid=0 auid=0 ses=8 subj=system_u:system_r:kernel_t:s0 op=collect_data cause=failed(directio) comm="syz-executor.2" name="file0" dev="sda1" ino=15750 res=0
 acct_on kernel/acct.c:207 [inline]
 __do_sys_acct kernel/acct.c:286 [inline]
 __se_sys_acct+0xca/0x7d0 kernel/acct.c:273
 __x64_sys_acct+0x2c/0x40 kernel/acct.c:273
 do_syscall_64+0xd0/0x4e0 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45cae9
Code: 8d b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f7df3811c78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a3
RAX: ffffffffffffffda RBX: 00000000004d9600 RCX: 000000000045cae9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000480
RBP: 000000000078bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 000000000000001c R14: 00000000004c2e88 R15: 00007f7df38126d4
==================================================================