Re: [syzbot] [kernfs?] WARNING: suspicious RCU usage in mas_start

[syzbot <syzbot+d79e205d463e603f47ff@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    f3a2439f20d9 Merge tag 'rproc-v6.3' of git://git.kernel.or..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=10a7dea8c80000
kernel config:  https://syzkaller.appspot.com/x/.config?x=dd442ddf29eaca0c
dashboard link: https://syzkaller.appspot.com/bug?extid=d79e205d463e603f47ff
compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12cfaf1cc80000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=149edbf0c80000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5892d3595732/disk-f3a2439f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b634456815c8/vmlinux-f3a2439f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/d6d82681f2ca/bzImage-f3a2439f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d79e205d463e603f47ff@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
6.2.0-syzkaller-12485-gf3a2439f20d9 #0 Not tainted
-----------------------------
lib/maple_tree.c:856 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
5 locks held by syz-executor322/5095:
 #0: ffff88802d3aa460 (sb_writers#8){.+.+}-{0:0}, at: vfs_write+0x26d/0xbb0 fs/read_write.c:580
 #1: ffff888023176c88 (&of->mutex){+.+.}-{3:3}, at: kernfs_fop_write_iter+0x1eb/0x4f0 fs/kernfs/file.c:325
 #2: ffff8881451a7490 (kn->active#47){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x20f/0x4f0 fs/kernfs/file.c:326
 #3: ffffffff8d214988 (ksm_thread_mutex){+.+.}-{3:3}, at: run_store+0x122/0xb10 mm/ksm.c:2953
 #4: ffff888025e8c998 (&mm->mmap_lock){++++}-{3:3}, at: mmap_read_lock include/linux/mmap_lock.h:117 [inline]
 #4: ffff888025e8c998 (&mm->mmap_lock){++++}-{3:3}, at: unmerge_and_remove_all_rmap_items mm/ksm.c:990 [inline]
 #4: ffff888025e8c998 (&mm->mmap_lock){++++}-{3:3}, at: run_store+0x2db/0xb10 mm/ksm.c:2959

stack backtrace:
CPU: 0 PID: 5095 Comm: syz-executor322 Not tainted 6.2.0-syzkaller-12485-gf3a2439f20d9 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/16/2023
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
 lockdep_rcu_suspicious+0x220/0x340 kernel/locking/lockdep.c:6599
 mas_root lib/maple_tree.c:856 [inline]
 mas_start+0x2c1/0x440 lib/maple_tree.c:1357
 mas_state_walk lib/maple_tree.c:3838 [inline]
 mas_walk+0x33/0x180 lib/maple_tree.c:5052
 mas_find+0x1e9/0x240 lib/maple_tree.c:6030
 vma_next include/linux/mm.h:745 [inline]
 unmerge_and_remove_all_rmap_items mm/ksm.c:991 [inline]
 run_store+0x2f9/0xb10 mm/ksm.c:2959
 kernfs_fop_write_iter+0x3a6/0x4f0 fs/kernfs/file.c:334
 call_write_iter include/linux/fs.h:1851 [inline]
 new_sync_write fs/read_write.c:491 [inline]
 vfs_write+0x7b2/0xbb0 fs/read_write.c:584
 ksys_write+0x1a0/0x2c0 fs/read_write.c:637
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7fe864e99e49
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 14 00 00 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 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc685fb328 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000000f4240 RCX: 00007fe864e99e49
RDX: 0000000000000002 RSI: 0000000020000000 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000246 R12: 000000000000d884
R13: 00007ffc685fb33c R14: 00007ffc685fb350 R15: 00007ffc685fb340
 </TASK>