CVE-2024-26727

In the Linux kernel, the following vulnerability has been resolved: btrfs:
do not ASSERT() if the newly created subvolume already got read [BUG] There
is a syzbot crash, triggered by the ASSERT() during subvolume creation:
assertion failed: !anon_dev, in fs/btrfs/disk-io.c:1319 ------------[ cut
here ]------------ kernel BUG at fs/btrfs/disk-io.c:1319! invalid opcode:
0000 [#1] PREEMPT SMP KASAN RIP: 0010:btrfs_get_root_ref.part.0+0x9aa/0xa60
<TASK> btrfs_get_new_fs_root+0xd3/0xf0 create_subvol+0xd02/0x1650
btrfs_mksubvol+0xe95/0x12b0 __btrfs_ioctl_snap_create+0x2f9/0x4f0
btrfs_ioctl_snap_create+0x16b/0x200 btrfs_ioctl+0x35f0/0x5cf0
__x64_sys_ioctl+0x19d/0x210 do_syscall_64+0x3f/0xe0
entry_SYSCALL_64_after_hwframe+0x63/0x6b —[ end trace 0000000000000000
]— [CAUSE] During create_subvol(), after inserting root item for the
newly created subvolume, we would trigger btrfs_get_new_fs_root() to get
the btrfs_root of that subvolume. The idea here is, we have preallocated an
anonymous device number for the subvolume, thus we can assign it to the new
subvolume. But there is really nothing preventing things like backref walk
to read the new subvolume. If that happens before we call
btrfs_get_new_fs_root(), the subvolume would be read out, with a new
anonymous device number assigned already. In that case, we would trigger
ASSERT(), as we really expect no one to read out that subvolume (which is
not yet accessible from the fs). But things like backref walk is still
possible to trigger the read on the subvolume. Thus our assumption on the
ASSERT() is not correct in the first place. [FIX] Fix it by removing the
ASSERT(), and just free the @anon_dev, reset it to 0, and continue. If the
subvolume tree is read out by something else, it should have already get a
new anon_dev assigned thus we only need to free the preallocated one.