CVE-2024-26792: In the Linux kernel, the following vulnerability has been resolved:  btrfs: fix double free of anonymous device after snapshot creation failure  When creating a snapshot we may do a double free of an anonymous device in case there's an error committing the transaction. The second free may result in freeing an anonymous device number that was allocated by some other subsystem in the kernel or another btrfs filesystem.  The steps that lead to this:  1) At ioctl.c:create_snapshot() we allocate an anonymous device number    and assign it to pending_snapshot->anon_dev;  2) Then we call btrfs_commit_transaction() and end up at    transaction.c:create_pending_snapshot();  3) There we call btrfs_get_new_fs_root() and pass it the anonymous device    number stored in pending_snapshot->anon_dev;  4) btrfs_get_new_fs_root() frees that anonymous device number because    btrfs_lookup_fs_root() returned a root - someone else did a lookup    of the new root already, which could some task doing backref walking;  5) After that some error happens in the transaction commit path, and at    ioctl.c:create_snapshot() we jump to the 'fail' label, and after    that we free again the same anonymous device number, which in the    meanwhile may have been reallocated somewhere else, because    pending_snapshot->anon_dev still has the same value as in step 1.  Recently syzbot ran into this and reported the following trace:    ------------[ cut here ]------------   ida_free called for id=51 which is not allocated.   WARNING: CPU: 1 PID: 31038 at lib/idr.c:525 ida_free+0x370/0x420 lib/idr.c:525   Modules linked in:   CPU: 1 PID: 31038 Comm: syz-executor.2 Not tainted 6.8.0-rc4-syzkaller-00410-gc02197fc9076 #0   Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024   RIP: 0010:ida_free+0x370/0x420 lib/idr.c:525   Code: 10 42 80 3c 28 (...)   RSP: 0018:ffffc90015a67300 EFLAGS: 00010246   RAX: be5130472f5dd000 RBX: 0000000000000033 RCX: 0000000000040000   RDX: ffffc90009a7a000 RSI: 000000000003ffff RDI: 0000000000040000   RBP: ffffc90015a673f0 R08: ffffffff81577992 R09: 1ffff92002b4cdb4   R10: dffffc0000000000 R11: fffff52002b4cdb5 R12: 0000000000000246   R13: dffffc0000000000 R14: ffffffff8e256b80 R15: 0000000000000246   FS:  00007fca3f4b46c0(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033   CR2: 00007f167a17b978 CR3: 000000001ed26000 CR4: 0000000000350ef0   Call Trace:    <TASK>    btrfs_get_root_ref+0xa48/0xaf0 fs/btrfs/disk-io.c:1346    create_pending_snapshot+0xff2/0x2bc0 fs/btrfs/transaction.c:1837    create_pending_snapshots+0x195/0x1d0 fs/btrfs/transaction.c:1931    btrfs_commit_transaction+0xf1c/0x3740 fs/btrfs/transaction.c:2404    create_snapshot+0x507/0x880 fs/btrfs/ioctl.c:848    btrfs_mksubvol+0x5d0/0x750 fs/btrfs/ioctl.c:998    btrfs_mksnapshot+0xb5/0xf0 fs/btrfs/ioctl.c:1044    __btrfs_ioctl_snap_create+0x387/0x4b0 fs/btrfs/ioctl.c:1306    btrfs_ioctl_snap_create_v2+0x1ca/0x400 fs/btrfs/ioctl.c:1393    btrfs_ioctl+0xa74/0xd40    vfs_ioctl fs/ioctl.c:51 [inline]    __do_sys_ioctl fs/ioctl.c:871 [inline]    __se_sys_ioctl+0xfe/0x170 fs/ioctl.c:857    do_syscall_64+0xfb/0x240    entry_SYSCALL_64_after_hwframe+0x6f/0x77   RIP: 0033:0x7fca3e67dda9   Code: 28 00 00 00 (...)   RSP: 002b:00007fca3f4b40c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010   RAX: ffffffffffffffda RBX: 00007fca3e7abf80 RCX: 00007fca3e67dda9   RDX: 00000000200005c0 RSI: 0000000050009417 RDI: 0000000000000003   RBP: 00007fca3e6ca47a R08: 0000000000000000 R09: 0000000000000000   R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000   R13: 000000000000000b R14: 00007fca3e7abf80 R15: 00007fff6bf95658    </TASK>  Where we get an explicit message where we attempt to free an anonymous device number that is not currently allocated. It happens in a different code path from the example below, at btrfs_get_root_ref(), so this change may not fix the case triggered by sy ---truncated---