On 2025-04-04 14:54:50, Blaise Boscaccy wrote:
> +static int hornet_verify_lskel(struct bpf_prog *prog, struct hornet_maps
> *maps,
> + void *sig, size_t sig_len)
> +{
> + int fd;
> + u32 i;
> + void *buf;
> + void *new;
> + size_t buf_sz;
> + struct bpf_map *map;
> + int err = 0;
> + int key = 0;
> + union bpf_attr attr = {0};
> +
> + buf = kmalloc_array(prog->len, sizeof(struct bpf_insn), GFP_KERNEL);
> + if (!buf)
> + return -ENOMEM;
> + buf_sz = prog->len * sizeof(struct bpf_insn);
> + memcpy(buf, prog->insnsi, buf_sz);
> +
> + for (i = 0; i < maps->used_map_cnt; i++) {
> + err = copy_from_bpfptr_offset(&fd, maps->fd_array,
> + maps->used_idx[i] * sizeof(fd),
> + sizeof(fd));
> + if (err < 0)
> + continue;
> + if (fd < 1)
> + continue;
> +
> + map = bpf_map_get(fd);
I'm not very familiar with BPF map lifetimes but I'd assume we need to have a
corresponding bpf_map_put(map) before returning.
> + if (IS_ERR(map))
> + continue;
> +
> + /* don't allow userspace to change map data used for signature
> verification */
> + if (!map->frozen) {
> + attr.map_fd = fd;
> + err = kern_sys_bpf(BPF_MAP_FREEZE, &attr, sizeof(attr));
> + if (err < 0)
> + goto out;
> + }
> +
> + new = krealloc(buf, buf_sz + map->value_size, GFP_KERNEL);
> + if (!new) {
> + err = -ENOMEM;
> + goto out;
> + }
> + buf = new;
> + new = map->ops->map_lookup_elem(map, &key);
> + if (!new) {
> + err = -ENOENT;
> + goto out;
> + }
> + memcpy(buf + buf_sz, new, map->value_size);
> + buf_sz += map->value_size;
> + }
> +
> + err = verify_pkcs7_signature(buf, buf_sz, sig, sig_len,
> + VERIFY_USE_SECONDARY_KEYRING,
> + VERIFYING_EBPF_SIGNATURE,
> + NULL, NULL);
> +out:
> + kfree(buf);
> + return err;
> +}
> +
> +static int hornet_check_binary(struct bpf_prog *prog, union bpf_attr *attr,
> + struct hornet_maps *maps)
> +{
> + struct file *file = get_task_exe_file(current);
We should handle get_task_exe_file() returning NULL. I don't think it is likely
to happen when passing `current` but kernel_read_file() doesn't protect against
it and we'll have a NULL pointer dereference when it calls file_inode(NULL).
> + const unsigned long markerlen = sizeof(EBPF_SIG_STRING) - 1;
> + void *buf = NULL;
> + size_t sz = 0, sig_len, prog_len, buf_sz;
> + int err = 0;
> + struct module_signature sig;
> +
> + buf_sz = kernel_read_file(file, 0, &buf, INT_MAX, &sz, READING_EBPF);
We are leaking buf in this function. kernel_read_file() allocates the memory
for us but we never kfree(buf).
> + fput(file);
> + if (!buf_sz)
> + return -1;
> +
> + prog_len = buf_sz;
> +
> + if (prog_len > markerlen &&
> + memcmp(buf + prog_len - markerlen, EBPF_SIG_STRING, markerlen) == 0)
> + prog_len -= markerlen;
Why is the marker optional? Looking at module_sig_check(), which verifies the
signature on kernel modules, I see that it refuses to proceed if the marker is
not found. Should we do the same and refuse to operate on any unexpected input?
> +
> + memcpy(&sig, buf + (prog_len - sizeof(sig)), sizeof(sig));
We should make sure that prog_len is larger than sizeof(sig) prior to this
memcpy(). It is probably not a real issue in practice but it would be good to
ensure that we can't be tricked to copy and operate on any bytes proceeding
buf.
Tyler
> + sig_len = be32_to_cpu(sig.sig_len);
> + prog_len -= sig_len + sizeof(sig);
> +
> + err = mod_check_sig(&sig, prog->len * sizeof(struct bpf_insn), "ebpf");
> + if (err)
> + return err;
> + return hornet_verify_lskel(prog, maps, buf + prog_len, sig_len);
> +}
