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

On Tue, Sep 05, 2017 at 02:59:38PM -0700, Chenbo Feng wrote:

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

quoted

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.

iirc the idea to have read only maps was brought up in the past
(unfortunately no one took a stab at implementing it), but imo
that's better then relying on lsm to provide such restriction
and more secure, since even if you disable map_update via lsm,
the user can craft a program just to udpate the map from it.
For bpffs we already test for inode_permission(inode, MAY_WRITE);
during BPF_OBJ_GET command and we can extend this facility further.
Also we can allow dropping 'write' permissions from the map
(internally implemented via flag in struct bpf_map), so
update/delete operations won't be allowed.
This flag will be checked by syscall during map_update/delete
and by the verifier if such read-only map is used by the program
being loaded, so map_update/helpers won't be allowed in
such program.
Would also be good to support read-only programs as well
(progs that are read-only from kernel state point of view)
They won't be able to modify packets, maps, etc.

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