Message-ID: <ec6784ed-8722-4695-980a-4400d4e7bd1a@gmx.com>
Date: Mon, 30 Dec 2024 19:59:41 +1030
From: Qu Wenruo <quwenruo.btrfs@....com>
To: linux-fsdevel@...r.kernel.org, linux-btrfs <linux-btrfs@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>
Subject: mnt_list corruption triggered during btrfs/326

Hi,

Although I know it's triggered from btrfs, but the mnt_list handling is
out of btrfs' control, so I'm here asking for some help.

[BUG]
With CONFIG_DEBUG_LIST and CONFIG_BUG_ON_DATA_CORRUPTION, and an
upstream 6.13-rc kernel, which has commit 951a3f59d268 ("btrfs: fix
mount failure due to remount races"), I can hit the following crash,
with varied frequency (from 1/4 to hundreds runs no crash):

[  303.356328] BTRFS: device fsid 6fd8eb6f-1ea5-40aa-9857-05c64efe6d43
devid 1 transid 9 /dev/mapper/test-scratch1 (253:2) scanned by mount
(358060)
[  303.358614] BTRFS info (device dm-2): first mount of filesystem
6fd8eb6f-1ea5-40aa-9857-05c64efe6d43
[  303.359475] BTRFS info (device dm-2): using crc32c (crc32c-intel)
checksum algorithm
[  303.360134] BTRFS info (device dm-2): using free-space-tree
[  313.264317] list_del corruption, ffff8fd48a7b2c90->prev is NULL
[  313.264966] ------------[ cut here ]------------
[  313.265402] kernel BUG at lib/list_debug.c:54!
[  313.265847] Oops: invalid opcode: 0000 [#1] PREEMPT SMP
[  313.266335] CPU: 4 UID: 0 PID: 370457 Comm: mount Kdump: loaded Not
tainted 6.13.0-rc4-custom+ #8
[  313.267252] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
Arch Linux 1.16.3-1-1 04/01/2014
[  313.268147] RIP: 0010:__list_del_entry_valid_or_report.cold+0x6d/0x6f
[  313.268777] Code: 05 77 a0 e8 4b 10 fd ff 0f 0b 48 89 fe 48 c7 c7 90
05 77 a0 e8 3a 10 fd ff 0f 0b 48 89 fe 48 c7 c7 60 05 77 a0 e8 29 10 fd
ff <0f> 0b 4c 89 ea be 01 00 00 00 4c 89 44 24 48 48 c7 c7 20 7c 2b a1
[  313.270493] RSP: 0018:ffffa7620d2b3a38 EFLAGS: 00010246
[  313.270960] RAX: 0000000000000033 RBX: ffff8fd48a7b2c00 RCX:
0000000000000000
[  313.271565] RDX: 0000000000000000 RSI: ffff8fd5f7c21900 RDI:
ffff8fd5f7c21900
[  313.272226] RBP: ffff8fd48a7b2c00 R08: 0000000000000000 R09:
0000000000000000
[  313.272895] R10: 74707572726f6320 R11: 6c65645f7473696c R12:
ffffa7620d2b3a58
[  313.273521] R13: ffff8fd48a7b2c00 R14: 0000000000000000 R15:
ffff8fd48a7b2c90
[  313.274138] FS:  00007f04740d4800(0000) GS:ffff8fd5f7c00000(0000)
knlGS:0000000000000000
[  313.274864] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  313.275392] CR2: 00007f0473ff6000 CR3: 000000011a8eb000 CR4:
0000000000750ef0
[  313.276084] PKRU: 55555554
[  313.276327] Call Trace:
[  313.276551]  <TASK>
[  313.276752]  ? __die_body.cold+0x19/0x28
[  313.277102]  ? die+0x2e/0x50
[  313.277699]  ? do_trap+0xc6/0x110
[  313.278033]  ? do_error_trap+0x6a/0x90
[  313.278401]  ? __list_del_entry_valid_or_report.cold+0x6d/0x6f
[  313.278941]  ? exc_invalid_op+0x50/0x60
[  313.279308]  ? __list_del_entry_valid_or_report.cold+0x6d/0x6f
[  313.279850]  ? asm_exc_invalid_op+0x1a/0x20
[  313.280241]  ? __list_del_entry_valid_or_report.cold+0x6d/0x6f
[  313.280777]  ? __list_del_entry_valid_or_report.cold+0x6d/0x6f
[  313.281285]  umount_tree+0xed/0x3c0
[  313.281589]  put_mnt_ns+0x51/0x90
[  313.281886]  mount_subtree+0x92/0x130
[  313.282205]  btrfs_get_tree+0x343/0x6b0 [btrfs]
[  313.282785]  vfs_get_tree+0x23/0xc0
[  313.283089]  vfs_cmd_create+0x59/0xd0
[  313.283406]  __do_sys_fsconfig+0x4eb/0x6b0
[  313.283764]  do_syscall_64+0x82/0x160
[  313.284085]  ? syscall_exit_to_user_mode_prepare+0x15a/0x190
[  313.284598]  ? __fs_parse+0x68/0x1b0
[  313.284929]  ? btrfs_parse_param+0x64/0x870 [btrfs]
[  313.285381]  ? vfs_parse_fs_param_source+0x20/0x90
[  313.285825]  ? __do_sys_fsconfig+0x1b8/0x6b0
[  313.286215]  ? syscall_exit_to_user_mode_prepare+0x15a/0x190
[  313.286719]  ? syscall_exit_to_user_mode+0x10/0x200
[  313.287151]  ? do_syscall_64+0x8e/0x160
[  313.287498]  ? vfs_fstatat+0x75/0xa0
[  313.287835]  ? __do_sys_newfstatat+0x56/0x90
[  313.288240]  ? syscall_exit_to_user_mode_prepare+0x15a/0x190
[  313.288749]  ? syscall_exit_to_user_mode+0x10/0x200
[  313.289188]  ? do_syscall_64+0x8e/0x160
[  313.289544]  ? do_syscall_64+0x8e/0x160
[  313.289892]  ? do_syscall_64+0x8e/0x160
[  313.290253]  ? syscall_exit_to_user_mode+0x10/0x200
[  313.290692]  ? do_syscall_64+0x8e/0x160
[  313.291034]  ? exc_page_fault+0x7e/0x180
[  313.291380]  entry_SYSCALL_64_after_hwframe+0x4b/0x53
[  313.291845] RIP: 0033:0x7f04742a919e
[  313.292182] Code: 73 01 c3 48 8b 0d 72 3c 0f 00 f7 d8 64 89 01 48 83
c8 ff c3 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 49 89 ca b8 af 01 00 00 0f
05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 42 3c 0f 00 f7 d8 64 89 01 48
[  313.293830] RSP: 002b:00007ffc3df08df8 EFLAGS: 00000246 ORIG_RAX:
00000000000001af
[  313.294529] RAX: ffffffffffffffda RBX: 000056407e37aa00 RCX:
00007f04742a919e
[  313.295201] RDX: 0000000000000000 RSI: 0000000000000006 RDI:
0000000000000003
[  313.295864] RBP: 00007ffc3df08f40 R08: 0000000000000000 R09:
0000000000000001
[  313.296602] R10: 0000000000000000 R11: 0000000000000246 R12:
00007f0474423b00
[  313.297416] R13: 0000000000000000 R14: 000056407e37cbe0 R15:
00007f0474418561
[  313.298242]  </TASK>
[  313.298832] Modules linked in: nft_fib_inet nft_fib_ipv4 nft_fib_ipv6
nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct
nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
nf_tables binfmt_misc btrfs xor raid6_pq zstd_compress iTCO_wdt
intel_pmc_bxt iTCO_vendor_support i2c_i801 i2c_smbus virtio_net joydev
net_failover lpc_ich virtio_balloon failover loop dm_multipath nfnetlink
vsock_loopback vmw_vsock_virtio_transport_common vsock zram
crct10dif_pclmul crc32_pclmul crc32c_intel polyval_clmulni
polyval_generic ghash_clmulni_intel virtio_console sha512_ssse3
sha256_ssse3 bochs sha1_ssse3 virtio_blk serio_raw scsi_dh_rdac
scsi_dh_emc scsi_dh_alua fuse qemu_fw_cfg
[  313.304504] Dumping ftrace buffer:
[  313.304876]    (ftrace buffer empty)

[EARLY ANALYZE]

The offending line is the list_move() call inside unmount_tree().

With crash core dump, the offending mnt_list is totally corrupted:

crash> struct list_head ffff8fd48a7b2c90
struct list_head {
   next = 0x1,
   prev = 0x0
}

umount_tree() should be protected by @mount_lock seqlock, and
@namespace_sem rwsem.

I also checked other mnt_list users:

- commit_tree()
- do_umount()
- copy_tree()

They all hold write @mount_lock at least.

The only caller doesn't hold @mount_lock is iterate_mounts() but that's
only called from audit, and I'm not sure if audit is even involved in
this case.

So I ran out of ideas why this mnt_list can even happen.

Even if it's some btrfs' abuse, all mnt_list users are properly
protected thus it should not lead to such list corruption.

Any advice would be appreciated.

Thanks,
Qu

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