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SubjectKASAN: null-ptr-deref Write in get_block
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Hello,

syzbot found the following crash on:

HEAD commit: fb33c651 Linux 5.6-rc6
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=157d4b45e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=9f894bd92023de02
dashboard link: https://syzkaller.appspot.com/bug?extid=4a88b2b9dc280f47baf4
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=127acd55e00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15f82345e00000

Bisection is inconclusive: the bug happens on the oldest tested release.

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=10e8d8d3e00000
final crash: https://syzkaller.appspot.com/x/report.txt?x=12e8d8d3e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=14e8d8d3e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+4a88b2b9dc280f47baf4@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in test_and_set_bit_lock include/asm-generic/bitops/instrumented-lock.h:55 [inline]
BUG: KASAN: null-ptr-deref in trylock_buffer include/linux/buffer_head.h:359 [inline]
BUG: KASAN: null-ptr-deref in lock_buffer include/linux/buffer_head.h:365 [inline]
BUG: KASAN: null-ptr-deref in alloc_branch fs/minix/itree_common.c:88 [inline]
BUG: KASAN: null-ptr-deref in get_block+0x657/0x1380 fs/minix/itree_common.c:191
Write of size 8 at addr 0000000000000000 by task syz-executor110/9537

CPU: 0 PID: 9537 Comm: syz-executor110 Not tainted 5.6.0-rc6-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+0x188/0x20d lib/dump_stack.c:118
__kasan_report.cold+0x5/0x32 mm/kasan/report.c:510
kasan_report+0xe/0x20 mm/kasan/common.c:641
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x128/0x190 mm/kasan/generic.c:192
test_and_set_bit_lock include/asm-generic/bitops/instrumented-lock.h:55 [inline]
trylock_buffer include/linux/buffer_head.h:359 [inline]
lock_buffer include/linux/buffer_head.h:365 [inline]
alloc_branch fs/minix/itree_common.c:88 [inline]
get_block+0x657/0x1380 fs/minix/itree_common.c:191
minix_get_block+0xe5/0x110 fs/minix/inode.c:376
__block_write_begin_int+0x490/0x1b00 fs/buffer.c:2008
__block_write_begin fs/buffer.c:2058 [inline]
block_write_begin+0x58/0x2e0 fs/buffer.c:2117
minix_write_begin+0x35/0xe0 fs/minix/inode.c:412
generic_perform_write+0x20a/0x4e0 mm/filemap.c:3287
__generic_file_write_iter+0x24c/0x610 mm/filemap.c:3416
generic_file_write_iter+0x3f0/0x62d mm/filemap.c:3448
call_write_iter include/linux/fs.h:1902 [inline]
new_sync_write+0x49c/0x700 fs/read_write.c:483
__vfs_write+0xc9/0x100 fs/read_write.c:496
vfs_write+0x262/0x5c0 fs/read_write.c:558
ksys_write+0x127/0x250 fs/read_write.c:611
do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x44b389
Code: fd ca 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 cb ca fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f4e9f99ace8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000006ddc38 RCX: 000000000044b389
RDX: 000000000000fdef RSI: 00000000200004c0 RDI: 0000000000000005
RBP: 00000000006ddc30 R08: 0000000000000012 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006ddc3c
R13: 00007ffcd0e441ef R14: 00007f4e9f99b9c0 R15: 0000000000000001
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

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