CVE-2024-26644: In the Linux kernel, the following vulnerability has been resolved:btrfs: don't abort filesystem when attempting to snapshot deleted subvolumeIf the source file descriptor to the snapshot ioctl refers to a deletedsubvolume, 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 entryThis happens because create_pending_snapshot() initializes the new rootitem 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() thenfinds the root and returns -ENOENT if refs == 0, which causescreate_pending_snapshot() to abort.Fix it by checking the source root's refs before attempting thesnapshot, but after locking subvol_sem to avoid racing with deletion.