Re: [PATCH 2/3] security: bpf: Add eBPF LSM hooks and security field to eBPF map

On Thu, Aug 31, 2017 at 7:05 PM, Alexei Starovoitov
[off-list ref] wrote:

On Thu, Aug 31, 2017 at 01:56:34PM -0700, Chenbo Feng wrote:

quoted

From: Chenbo Feng <redacted>

Introduce a pointer into struct bpf_map to hold the security information
about the map. The actual security struct varies based on the security
models implemented. Place the LSM hooks before each of the unrestricted
eBPF operations, the map_update_elem and map_delete_elem operations are
checked by security_map_modify. The map_lookup_elem and map_get_next_key
operations are checked by securtiy_map_read.

Signed-off-by: Chenbo Feng <redacted>

...

quoted

@@ -410,6 +418,10 @@ static int map_lookup_elem(union bpf_attr *attr)
      if (IS_ERR(map))
              return PTR_ERR(map);

+     err = security_map_read(map);
+     if (err)
+             return -EACCES;
+
      key = memdup_user(ukey, map->key_size);
      if (IS_ERR(key)) {
              err = PTR_ERR(key);

@@ -490,6 +502,10 @@ static int map_update_elem(union bpf_attr *attr)
      if (IS_ERR(map))
              return PTR_ERR(map);

+     err = security_map_modify(map);

I don't feel these extra hooks are really thought through.
With such hook you'll disallow map_update for given map. That's it.
The key/values etc won't be used in such security decision.
In such case you don't need such hooks in update/lookup at all.
Only in map_creation and object_get calls where FD can be received.
In other words I suggest to follow standard unix practices:
Do permissions checks in open() and allow read/write() if FD is valid.
Same here. Do permission checks in prog_load/map_create/obj_pin/get
and that will be enough to jail bpf subsystem.
bpf cmds that need to be fast (like lookup and update) should not
have security hooks.

I do think we want to distinguish between read/write (or read/modify)
for these objects. Essentially, we want to implement the example
described in patch 0/3 where eBPF objects can be passed to less
privileged processes which can read, but not modify the map. What
would be the best way to do this? Add a mode field to the bpf_map
struct?