BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: test_and_set_bit arch/x86/include/asm/bitops.h:220 [inline]
IP: test_and_set_bit_lock arch/x86/include/asm/bitops.h:233 [inline]
IP: trylock_buffer include/linux/buffer_head.h:359 [inline]
IP: lock_buffer include/linux/buffer_head.h:365 [inline]
IP: alloc_branch fs/minix/itree_common.c:88 [inline]
IP: get_block+0x6a7/0x10f0 fs/minix/itree_common.c:191
PGD 9f739067 P4D 9f739067 PUD 9f762067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Modules linked in:
CPU: 1 PID: 17025 Comm: syz-executor.3 Not tainted 4.14.176-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff888043d0a4c0 task.stack: ffff888040d58000
RIP: 0010:test_and_set_bit arch/x86/include/asm/bitops.h:220 [inline]
RIP: 0010:test_and_set_bit_lock arch/x86/include/asm/bitops.h:233 [inline]
RIP: 0010:trylock_buffer include/linux/buffer_head.h:359 [inline]
RIP: 0010:lock_buffer include/linux/buffer_head.h:365 [inline]
RIP: 0010:alloc_branch fs/minix/itree_common.c:88 [inline]
RIP: 0010:get_block+0x6a7/0x10f0 fs/minix/itree_common.c:191
RSP: 0018:ffff888040d5f808 EFLAGS: 00010246
RAX: 0000000000000007 RBX: ffff888040d5f910 RCX: 1ffffffff10279bc
RDX: 0000000000000000 RSI: ffffffff87d842e0 RDI: ffff888043d0a4e4
RBP: ffff888040d5f988 R08: 0000000000000001 R09: 0000000000000003
R10: ffff888043d0adb8 R11: ffff888043d0a4c0 R12: ffff888040d5f8d4
R13: ffff888083698140 R14: dffffc0000000000 R15: 0000000000000000
FS:  00007f3499dfb700(0000) GS:ffff8880aeb00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000008776c000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 minix_get_block+0xd6/0x100 fs/minix/inode.c:379
 __block_write_begin_int+0x337/0x1030 fs/buffer.c:2027
 __block_write_begin fs/buffer.c:2077 [inline]
 block_write_begin+0x58/0x260 fs/buffer.c:2136
 minix_write_begin+0x35/0xc0 fs/minix/inode.c:415
 generic_perform_write+0x1c9/0x420 mm/filemap.c:3047
 __generic_file_write_iter+0x227/0x590 mm/filemap.c:3172
 generic_file_write_iter+0x2fa/0x650 mm/filemap.c:3200
 call_write_iter include/linux/fs.h:1778 [inline]
 new_sync_write fs/read_write.c:469 [inline]
 __vfs_write+0x44e/0x630 fs/read_write.c:482
 vfs_write+0x192/0x4e0 fs/read_write.c:544
 SYSC_write fs/read_write.c:590 [inline]
 SyS_write+0xf2/0x210 fs/read_write.c:582
 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x45c889
RSP: 002b:00007f3499dfac78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f3499dfb6d4 RCX: 000000000045c889
RDX: 000000000000ff08 RSI: 0000000020000240 RDI: 0000000000000003
RBP: 000000000076bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000cde R14: 00000000004cf06a R15: 000000000076bf0c
Code: ca 08 00 00 49 8b bf 28 01 00 00 b9 08 00 00 00 e8 af 04 c6 ff 31 d2 be 6c 01 00 00 48 c7 c7 60 f1 b9 86 49 89 c7 e8 a9 42 6d ff <f0> 41 0f ba 2f 02 0f 83 1f fe ff ff e8 e8 2d 89 ff 4c 89 ff e8 
RIP: test_and_set_bit arch/x86/include/asm/bitops.h:220 [inline] RSP: ffff888040d5f808
RIP: test_and_set_bit_lock arch/x86/include/asm/bitops.h:233 [inline] RSP: ffff888040d5f808
RIP: trylock_buffer include/linux/buffer_head.h:359 [inline] RSP: ffff888040d5f808
RIP: lock_buffer include/linux/buffer_head.h:365 [inline] RSP: ffff888040d5f808
RIP: alloc_branch fs/minix/itree_common.c:88 [inline] RSP: ffff888040d5f808
RIP: get_block+0x6a7/0x10f0 fs/minix/itree_common.c:191 RSP: ffff888040d5f808
CR2: 0000000000000000
---[ end trace 19307af31dd86c48 ]---