btrfs: don't abort filesystem when attempting to snapshot deleted subvolume

In the Linux kernel, the following vulnerability has been resolved:

btrfs: don't abort filesystem when attempting to snapshot deleted subvolume

If the source file descriptor to the snapshot ioctl refers to a deleted

subvolume, we get the following abort:

BTRFS: Transaction aborted (error -2)

WARNING: CPU: 0 PID: 833 at fs/btrfs/transaction.c:1875 create_pending_snapshot+0x1040/0x1190 [btrfs]

Modules linked in: pata_acpi btrfs ata_piix libata scsi_mod virtio_net blake2b_generic xor net_failover virtio_rng failover scsi_common rng_core raid6_pq libcrc32c

CPU: 0 PID: 833 Comm: t_snapshot_dele Not tainted 6.7.0-rc6 #2

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-1.fc39 04/01/2014

RIP: 0010:create_pending_snapshot+0x1040/0x1190 [btrfs]

RSP: 0018:ffffa09c01337af8 EFLAGS: 00010282

RAX: 0000000000000000 RBX: ffff9982053e7c78 RCX: 0000000000000027

RDX: ffff99827dc20848 RSI: 0000000000000001 RDI: ffff99827dc20840

RBP: ffffa09c01337c00 R08: 0000000000000000 R09: ffffa09c01337998

R10: 0000000000000003 R11: ffffffffb96da248 R12: fffffffffffffffe

R13: ffff99820535bb28 R14: ffff99820b7bd000 R15: ffff99820381ea80

FS: 00007fe20aadabc0(0000) GS:ffff99827dc00000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 0000559a120b502f CR3: 00000000055b6000 CR4: 00000000000006f0

Call Trace:

<TASK>

? create_pending_snapshot+0x1040/0x1190 [btrfs]

? __warn+0x81/0x130

? create_pending_snapshot+0x1040/0x1190 [btrfs]

? report_bug+0x171/0x1a0

? handle_bug+0x3a/0x70

? exc_invalid_op+0x17/0x70

? asm_exc_invalid_op+0x1a/0x20

? create_pending_snapshot+0x1040/0x1190 [btrfs]

? create_pending_snapshot+0x1040/0x1190 [btrfs]

create_pending_snapshots+0x92/0xc0 [btrfs]

btrfs_commit_transaction+0x66b/0xf40 [btrfs]

btrfs_mksubvol+0x301/0x4d0 [btrfs]

btrfs_mksnapshot+0x80/0xb0 [btrfs]

__btrfs_ioctl_snap_create+0x1c2/0x1d0 [btrfs]

btrfs_ioctl_snap_create_v2+0xc4/0x150 [btrfs]

btrfs_ioctl+0x8a6/0x2650 [btrfs]

? kmem_cache_free+0x22/0x340

? do_sys_openat2+0x97/0xe0

__x64_sys_ioctl+0x97/0xd0

do_syscall_64+0x46/0xf0

entry_SYSCALL_64_after_hwframe+0x6e/0x76

RIP: 0033:0x7fe20abe83af

RSP: 002b:00007ffe6eff1360 EFLAGS: 00000246 ORIG_RAX: 0000000000000010

RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00007fe20abe83af

RDX: 00007ffe6eff23c0 RSI: 0000000050009417 RDI: 0000000000000003

RBP: 0000000000000003 R08: 0000000000000000 R09: 00007fe20ad16cd0

R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000

R13: 00007ffe6eff13c0 R14: 00007fe20ad45000 R15: 0000559a120b6d58

</TASK>

---[ end trace 0000000000000000 ]---

BTRFS: error (device vdc: state A) in create_pending_snapshot:1875: errno=-2 No such entry

BTRFS info (device vdc: state EA): forced readonly

BTRFS warning (device vdc: state EA): Skipping commit of aborted transaction.

BTRFS: error (device vdc: state EA) in cleanup_transaction:2055: errno=-2 No such entry

This happens because create_pending_snapshot() initializes the new root

item as a copy of the source root item. This includes the refs field,

which is 0 for a deleted subvolume. The call to btrfs_insert_root()

therefore inserts a root with refs == 0. btrfs_get_new_fs_root() then

finds the root and returns -ENOENT if refs == 0, which causes

create_pending_snapshot() to abort.

Fix it by checking the source root's refs before attempting the

snapshot, but after locking subvol_sem to avoid racing with deletion.