ovl: fix double end_creating() on the casefold-mismatch path

In the Linux kernel, the following vulnerability has been resolved:

ovl: fix double end_creating() on the casefold-mismatch path

ovl_create_real() releases the new dentry twice when the casefold
consistency check fails. The S_IFDIR branch calls end_creating() and
sets err, then falls through to the common out: label which calls
end_creating() on the same dentry again:

case S_IFDIR:
newdentry = ovl_do_mkdir(ofs, dir, newdentry, attr->mode);
err = PTR_ERR_OR_ZERO(newdentry);
if (!err && ofs->casefold != ovl_dentry_casefolded(newdentry)) {
pr_warn_ratelimited(...);
end_creating(newdentry); /* first */
err = -EINVAL;
}
break;
...
if (err)
goto out;
...
out:
if (err) {
end_creating(newdentry); /* second, same dentry */
return ERR_PTR(err);
}

end_creating() is end_dirop(), which does inode_unlock() on the parent
and dput() on the dentry, so the parent directory's i_rwsem is unlocked
twice and the dentry is put twice. The second unlock releases a lock
that is not held, which is what wedges every later creation under that
parent, and the second dput() drops a reference that was never taken.

The branch was added by commit dfc7da402ccc ("ovl: Check for casefold
consistency when creating new dentries") as a bare dput(), which already
released the reference twice; commit fe497f0759e0 ("VFS: change
vfs_mkdir() to unlock on failure.") converted both sites to
end_creating(), adding the double unlock.

This is reachable by an unprivileged user. The casefold consistency of
the layers is validated at mount time in ovl_parse_layer(), and again on
every lookup in ovl_lookup_single(), but ofs->workdir is the internal
"work" subdirectory created inside the user-supplied workdir, and that
subdirectory is not re-checked. Marking it casefolded after the mount
therefore makes every ovl_create_temp() inherit the wrong state - and
that path reaches ovl_create_real() through ovl_start_creating_temp(),
which uses start_creating() with a generated name and so never runs the
lookup-time check.

unshare -Urm
mount -t tmpfs -o casefold=utf8-12.1.0 tmpfs mnt
mkdir -p mnt/lower/d mnt/upper mnt/work mnt/merged
mount -t overlay ovl -o lowerdir=mnt/lower,\
upperdir=mnt/upper,workdir=mnt/work mnt/merged
chattr +F mnt/work/work
mkdir mnt/merged/d/sub # directory copy-up

overlayfs: wrong inherited casefold (work/#5)

and the next copy-up blocks forever on the parent's i_rwsem:

mkdir D start_creating+0x65/0xb0
ovl_start_creating_temp+0xb0/0xe0 [overlay]
ovl_create_temp+0xa3/0x1d0 [overlay]
ovl_copy_up_one+0x1f1c/0x21c0 [overlay]
ovl_copy_up_flags+0xf5/0x140 [overlay]
ovl_create_object+0xb7/0x220 [overlay]
ovl_mkdir+0x23/0x40 [overlay]

Drop the end_creating() from the branch and let out: own the cleanup,
which is what every other error path in this function already does.