Re: [syzbot] [bcachefs?] KASAN: slab-use-after-free Read in bch2_copygc

[syzbot <syzbot+3168625f36f4a539237e@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    7482bb149b9f Merge branch 'for-next/core' into for-kernelci
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=12bf4a8c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3c06e3e2454512b3
dashboard link: https://syzkaller.appspot.com/bug?extid=3168625f36f4a539237e
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
userspace arch: arm64
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14f63f70580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16bf4a8c580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f623d741d651/disk-7482bb14.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/483e23ae71b1/vmlinux-7482bb14.xz
kernel image: https://storage.googleapis.com/syzbot-assets/79b5baaa1b50/Image-7482bb14.gz.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/f9688b789a98/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3168625f36f4a539237e@syzkaller.appspotmail.com

bcachefs (loop1): Detected missing backpointers in bucket 34, now have 1/128 with missing
Unable to handle kernel paging request at virtual address ffff60021a145c30
KASAN: probably wild-memory-access in range [0xffff0010d0a2e180-0xffff0010d0a2e187]
Mem abort info:
  ESR = 0x0000000096000005
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x05: level 1 translation fault
Data abort info:
  ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
  CM = 0, WnR = 0, TnD = 0, TagAccess = 0
  GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000000207144000
[ffff60021a145c30] pgd=0000000000000000, p4d=000000023397b003, pud=0000000000000000
Internal error: Oops: 0000000096000005 [#1]  SMP
Modules linked in:
CPU: 0 UID: 0 PID: 6717 Comm: bch-copygc/loop Not tainted 6.16.0-rc4-syzkaller-g7482bb149b9f #0 PREEMPT 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : generic_test_bit include/asm-generic/bitops/generic-non-atomic.h:128 [inline]
pc : bch2_bucket_bitmap_test fs/bcachefs/backpointers.h:194 [inline]
pc : bch2_bucket_is_movable fs/bcachefs/movinggc.c:78 [inline]
pc : bch2_copygc_get_buckets fs/bcachefs/movinggc.c:157 [inline]
pc : bch2_copygc+0xab4/0x3460 fs/bcachefs/movinggc.c:221
lr : rcu_read_unlock include/linux/rcupdate.h:873 [inline]
lr : class_rcu_destructor include/linux/rcupdate.h:1155 [inline]
lr : bch2_dev_tryget_noerror fs/bcachefs/sb-members.h:241 [inline]
lr : bch2_dev_tryget fs/bcachefs/sb-members.h:245 [inline]
lr : bch2_bucket_is_movable fs/bcachefs/movinggc.c:74 [inline]
lr : bch2_copygc_get_buckets fs/bcachefs/movinggc.c:157 [inline]
lr : bch2_copygc+0xa7c/0x3460 fs/bcachefs/movinggc.c:221
sp : ffff80009e0c74c0
x29: ffff80009e0c7920 x28: 0000008000000022 x27: ffff80009e0c76c8
x26: dfff800000000000 x25: ffff80009e0c7cf8 x24: 0000000000000000
x23: ffff0000f5890000 x22: 0000000000000000 x21: ffff0000f2d00000
x20: ffff0010d0a2e180
 x19: ffff0000df09cc28 x18: 00000000ffffffff
x17: ffff800093363000 x16: ffff80008aefcb18 x15: 0000000000000001
x14: 1fffe0001eb12008 x13: 0000000000000000 x12: 0000000000000000
x11: ffff60001eb12009 x10: 0000000000ff0100 x9 : 0000001000000000
x8 : 1fffe0021a145c30 x7 : ffff800082a0df54 x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000000 x3 : ffff800082a0edd4
x2 : 0000000000000001 x1 : 0000000000000080 x0 : ffff0000cc76bd00
Call trace:
 generic_test_bit include/asm-generic/bitops/generic-non-atomic.h:128 [inline] (P)
 bch2_bucket_bitmap_test fs/bcachefs/backpointers.h:194 [inline] (P)
 bch2_bucket_is_movable fs/bcachefs/movinggc.c:78 [inline] (P)
 bch2_copygc_get_buckets fs/bcachefs/movinggc.c:157 [inline] (P)
 bch2_copygc+0xab4/0x3460 fs/bcachefs/movinggc.c:221 (P)
 bch2_copygc_thread+0x898/0xd8c fs/bcachefs/movinggc.c:409
 kthread+0x5fc/0x75c kernel/kthread.c:464
 ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:847
Code: d343fd29 927da529 8b090114 d343fe88 (387a6908) 
---[ end trace 0000000000000000 ]---
----------------
Code disassembly (best guess):
   0:	d343fd29 	lsr	x9, x9, #3
   4:	927da529 	and	x9, x9, #0x1ffffffffff8
   8:	8b090114 	add	x20, x8, x9
   c:	d343fe88 	lsr	x8, x20, #3
* 10:	387a6908 	ldrb	w8, [x8, x26] <-- trapping instruction


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