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---