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.

Thanks for pointing out this. I agree we should only do checks on
creating and passing
the object from one processes to another. And if we can still
distinguish the read/write operation
when obtaining the file fd, that would be great. But that may require
us to add a new mode
field into bpf_map struct and change the attribute struct when doing
the bpf syscall. How do you
think about this approach? Or we can do simple checks for
bpf_obj_create and bpf_obj_use when
creating the object and passing the object respectively but this
solution cannot distinguish map modify and
read.