KASAN: null-ptr-deref Write in start_transaction

Status: fixed on 2021/03/10 01:48
Subsystems: btrfs
[Documentation on labels]
Fix commit: ecfdc08b8cc6 btrfs: remove dio iomap DSYNC workaround
First crash: 1326d, last: 1295d
Cause bisection: introduced by (bisect log) :
commit 4d004099a668c41522242aa146a38cc4eb59cb1e
Author: Peter Zijlstra <>
Date: Fri Oct 2 09:04:21 2020 +0000

  lockdep: Fix lockdep recursion

Crash: BUG: using __this_cpu_read() in preemptible code in trace_hardirqs_on (log)
Repro: C syz .config
Fix bisection: fixed by (bisect log) :
commit f30bed83426c5cb9fce6cabb3f7cc5a9d5428fcc
Author: Filipe Manana <>
Date: Fri Nov 13 11:24:17 2020 +0000

  btrfs: remove unnecessary attempt to drop extent maps after adding inline extent

Discussions (1)
Title Replies (including bot) Last reply
KASAN: null-ptr-deref Write in start_transaction 5 (7) 2021/01/08 14:35
Fix bisection attempts (2)
Created Duration User Patch Repo Result
2021/01/07 09:51 3h18m bisect fix upstream job log (1)
2020/12/07 11:13 15m bisect fix upstream job log (0) log

Sample crash report:
BUG: KASAN: null-ptr-deref in instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
BUG: KASAN: null-ptr-deref in atomic_fetch_add_relaxed include/asm-generic/atomic-instrumented.h:142 [inline]
BUG: KASAN: null-ptr-deref in __refcount_add include/linux/refcount.h:193 [inline]
BUG: KASAN: null-ptr-deref in __refcount_inc include/linux/refcount.h:250 [inline]
BUG: KASAN: null-ptr-deref in refcount_inc include/linux/refcount.h:267 [inline]
BUG: KASAN: null-ptr-deref in start_transaction+0x158/0x1170 fs/btrfs/transaction.c:541
Write of size 4 at addr 000000000000003a by task syz-executor154/8513

CPU: 1 PID: 8513 Comm: syz-executor154 Not tainted 5.10.0-rc2-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+0x107/0x163 lib/dump_stack.c:118
 __kasan_report mm/kasan/report.c:549 [inline]
 kasan_report.cold+0x5/0x37 mm/kasan/report.c:562
 check_memory_region_inline mm/kasan/generic.c:186 [inline]
 check_memory_region+0x13d/0x180 mm/kasan/generic.c:192
 instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
 atomic_fetch_add_relaxed include/asm-generic/atomic-instrumented.h:142 [inline]
 __refcount_add include/linux/refcount.h:193 [inline]
 __refcount_inc include/linux/refcount.h:250 [inline]
 refcount_inc include/linux/refcount.h:267 [inline]
 start_transaction+0x158/0x1170 fs/btrfs/transaction.c:541
 flush_space+0x1c0/0xf60 fs/btrfs/space-info.c:685
 priority_reclaim_metadata_space fs/btrfs/space-info.c:1154 [inline]
 handle_reserve_ticket fs/btrfs/space-info.c:1238 [inline]
 __reserve_bytes+0xd2c/0x1480 fs/btrfs/space-info.c:1403
 btrfs_reserve_metadata_bytes+0x75/0x260 fs/btrfs/space-info.c:1429
 btrfs_delalloc_reserve_metadata+0x261/0xb90 fs/btrfs/delalloc-space.c:332
 btrfs_buffered_write.isra.0+0x445/0xf10 fs/btrfs/file.c:1703
 __btrfs_direct_write fs/btrfs/file.c:1874 [inline]
 btrfs_file_write_iter+0xda6/0x16d0 fs/btrfs/file.c:2046
 call_write_iter include/linux/fs.h:1887 [inline]
 do_iter_readv_writev+0x46f/0x740 fs/read_write.c:740
 do_iter_write+0x188/0x670 fs/read_write.c:866
 vfs_writev+0x1aa/0x2e0 fs/read_write.c:939
 do_pwritev fs/read_write.c:1036 [inline]
 __do_sys_pwritev fs/read_write.c:1083 [inline]
 __se_sys_pwritev fs/read_write.c:1078 [inline]
 __x64_sys_pwritev+0x231/0x310 fs/read_write.c:1078
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
RIP: 0033:0x44d959
Code: 7d cb 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 4b cb fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f78b0d8bce8 EFLAGS: 00000246 ORIG_RAX: 0000000000000128
RAX: ffffffffffffffda RBX: 00000000006e1c28 RCX: 000000000044d959
RDX: 0000000000000001 RSI: 00000000200014c0 RDI: 0000000000000003
RBP: 00000000006e1c20 R08: 0000000000000020 R09: 0000000000000000
R10: 0000000000000002 R11: 0000000000000246 R12: 00000000006e1c2c
R13: 00007ffefe638f4f R14: 00007f78b0d8c9c0 R15: 20c49ba5e353f7cf

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2020/11/06 18:47 upstream 521b619acdc8 64069d48 .config console log report syz C ci-upstream-kasan-gce-root
* Struck through repros no longer work on HEAD.