Re: [syzbot] [bcachefs?] general protection fault in bch2_snapshot_tree_oldest_subvol

From: Kent Overstreet
Date: Fri Apr 11 2025 - 13:29:42 EST

On Fri, Apr 11, 2025 at 03:35:26PM +0000, Charalampos Mitrodimas wrote:
> syzbot <syzbot+baee8591f336cab0958b@xxxxxxxxxxxxxxxxxxxxxxxxx> writes:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: a2cc6ff5ec8f Merge tag 'firewire-updates-6.15' of git://gi..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12482fb0580000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=6fe3b5e6a2cb1cc2
> > dashboard link: https://syzkaller.appspot.com/bug?extid=baee8591f336cab0958b
> > compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10530be4580000
> > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13d6d404580000
> >
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-a2cc6ff5.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/c58c1555aab7/vmlinux-a2cc6ff5.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/61fb9d013359/bzImage-a2cc6ff5.xz
> > mounted in repro: https://storage.googleapis.com/syzbot-assets/86cf46d3a5d9/mount_0.gz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+baee8591f336cab0958b@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > done
> > bcachefs (loop0): accounting_read... done
> > bcachefs (loop0): alloc_read... done
> > bcachefs (loop0): snapshots_read... done
> > bcachefs (loop0): check_allocations...
> > Oops: general protection fault, probably for non-canonical address 0xdffffc0000000004: 0000 [#1] SMP KASAN NOPTI
> > KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
> > CPU: 0 UID: 0 PID: 5318 Comm: syz-executor394 Not tainted 6.14.0-syzkaller-12966-ga2cc6ff5ec8f #0 PREEMPT(full)
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> > RIP: 0010:bch2_snapshot_tree_oldest_subvol+0x1d3/0x6a0 fs/bcachefs/snapshot.c:400
> > Code: e6 e8 81 dd 36 fd 4c 39 e5 0f 86 c9 03 00 00 e8 13 db 36 fd 49 6b c4 38 49 01 c6 49 83 c6 18 49 83 c6 20 4c 89 f0 48 c1 e8 03 <42> 0f b6 04 28 84 c0 0f 85 c6 03 00 00 41 8b 2e 31 ff 89 ee e8 24
> > RSP: 0018:ffffc9000d46e020 EFLAGS: 00010202
> > RAX: 0000000000000004 RBX: 0000000000000001 RCX: ffff888000f98000
> > RDX: 0000000000000000 RSI: 00000000ffeb487f RDI: 0000000000000001
> > RBP: 0000000000000001 R08: ffffffff848c7c5f R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffeb487f
> > R13: dffffc0000000000 R14: 0000000000000020 R15: 000000000014b780
> > FS: 0000555584fe3380(0000) GS:ffff88808c599000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 000055835cf3c000 CR3: 0000000044318000 CR4: 0000000000352ef0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> > <TASK>
> > bch2_inum_snap_offset_err_msg_trans+0x374/0x680 fs/bcachefs/error.c:691
> > bch2_indirect_extent_missing_error+0x411/0x1290 fs/bcachefs/reflink.c:192
> > gc_trigger_reflink_p_segment fs/bcachefs/reflink.c:392 [inline]
> > __trigger_reflink_p+0x196c/0x1cc0 fs/bcachefs/reflink.c:432
> > bch2_trigger_reflink_p+0x299/0x380 fs/bcachefs/reflink.c:451
> > bch2_key_trigger fs/bcachefs/bkey_methods.h:88 [inline]
> > bch2_gc_mark_key+0x6bd/0x1180 fs/bcachefs/btree_gc.c:639
> > bch2_gc_btree fs/bcachefs/btree_gc.c:677 [inline]
> > bch2_gc_btrees fs/bcachefs/btree_gc.c:740 [inline]
> > bch2_check_allocations+0x1488/0x6ab0 fs/bcachefs/btree_gc.c:1042
> > bch2_run_recovery_pass+0xf0/0x1e0 fs/bcachefs/recovery_passes.c:226
> > bch2_run_recovery_passes+0x2ad/0xa90 fs/bcachefs/recovery_passes.c:285
> > bch2_fs_recovery+0x292a/0x3e20 fs/bcachefs/recovery.c:936
> > bch2_fs_start+0x2fb/0x610 fs/bcachefs/super.c:1060
> > bch2_fs_get_tree+0x113e/0x18f0 fs/bcachefs/fs.c:2253
> > vfs_get_tree+0x90/0x2b0 fs/super.c:1759
> > do_new_mount+0x2cf/0xb70 fs/namespace.c:3878
> > do_mount fs/namespace.c:4218 [inline]
> > __do_sys_mount fs/namespace.c:4429 [inline]
> > __se_sys_mount+0x38c/0x400 fs/namespace.c:4406
> > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > do_syscall_64+0xf3/0x230 arch/x86/entry/syscall_64.c:94
> > entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f22f815ae2a
> > Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb a6 e8 5e 04 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007ffd84c61a58 EFLAGS: 00000282 ORIG_RAX: 00000000000000a5
> > RAX: ffffffffffffffda RBX: 00007ffd84c61a70 RCX: 00007f22f815ae2a
> > RDX: 0000200000000040 RSI: 0000200000000000 RDI: 00007ffd84c61a70
> > RBP: 0000200000000000 R08: 00007ffd84c61ab0 R09: 0000000000005995
> > R10: 0000000000800001 R11: 0000000000000282 R12: 0000200000000040
> > R13: 0000000000000004 R14: 0000000000000003 R15: 00007ffd84c61ab0
> > </TASK>
> > Modules linked in:
> > ---[ end trace 0000000000000000 ]---
> > RIP: 0010:bch2_snapshot_tree_oldest_subvol+0x1d3/0x6a0 fs/bcachefs/snapshot.c:400
> > Code: e6 e8 81 dd 36 fd 4c 39 e5 0f 86 c9 03 00 00 e8 13 db 36 fd 49 6b c4 38 49 01 c6 49 83 c6 18 49 83 c6 20 4c 89 f0 48 c1 e8 03 <42> 0f b6 04 28 84 c0 0f 85 c6 03 00 00 41 8b 2e 31 ff 89 ee e8 24
> > RSP: 0018:ffffc9000d46e020 EFLAGS: 00010202
> > RAX: 0000000000000004 RBX: 0000000000000001 RCX: ffff888000f98000
> > RDX: 0000000000000000 RSI: 00000000ffeb487f RDI: 0000000000000001
> > RBP: 0000000000000001 R08: ffffffff848c7c5f R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffeb487f
> > R13: dffffc0000000000 R14: 0000000000000020 R15: 000000000014b780
> > FS: 0000555584fe3380(0000) GS:ffff88808c599000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000556b9781f0c8 CR3: 0000000044318000 CR4: 0000000000352ef0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > ----------------
> > Code disassembly (best guess):
> > 0: e6 e8 out %al,$0xe8
> > 2: 81 dd 36 fd 4c 39 sbb $0x394cfd36,%ebp
> > 8: e5 0f in $0xf,%eax
> > a: 86 c9 xchg %cl,%cl
> > c: 03 00 add (%rax),%eax
> > e: 00 e8 add %ch,%al
> > 10: 13 db adc %ebx,%ebx
> > 12: 36 fd ss std
> > 14: 49 6b c4 38 imul $0x38,%r12,%rax
> > 18: 49 01 c6 add %rax,%r14
> > 1b: 49 83 c6 18 add $0x18,%r14
> > 1f: 49 83 c6 20 add $0x20,%r14
> > 23: 4c 89 f0 mov %r14,%rax
> > 26: 48 c1 e8 03 shr $0x3,%rax
> > * 2a: 42 0f b6 04 28 movzbl (%rax,%r13,1),%eax <-- trapping instruction
> > 2f: 84 c0 test %al,%al
> > 31: 0f 85 c6 03 00 00 jne 0x3fd
> > 37: 41 8b 2e mov (%r14),%ebp
> > 3a: 31 ff xor %edi,%edi
> > 3c: 89 ee mov %ebp,%esi
> > 3e: e8 .byte 0xe8
> > 3f: 24 .byte 0x24
> >
> >
>
> #syz test
>
> diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
> index 0c65065b08ec..8862714b1806 100644
> --- a/fs/bcachefs/snapshot.c
> +++ b/fs/bcachefs/snapshot.c
> @@ -397,7 +397,11 @@ u32 bch2_snapshot_tree_oldest_subvol(struct bch_fs *c, u32 snapshot_root)
>
> rcu_read_lock();
> while (id) {
> - s = snapshot_t(c, id)->subvol;
> + const struct snapshot_t *snap = snapshot_t(c, id);
> + if (!snap)
> + break;

???

> +
> + s = snap->subvol;
>
> if (s && (!subvol || s < subvol))
> subvol = s;

Don't fix null ptr derefs like this, you're just papering over bugs.

If the snapshot isn't found that indicates snapshot tree corruption, and
at a minimum that needs to be flagged for repair.