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—