Re: [RFC v2 06/10] landlock: Add LSM hooks

[Andy Lutomirski]

On Tue, Aug 30, 2016 at 1:10 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>
> On 30/08/2016 20:56, Andy Lutomirski wrote:
>> On Aug 25, 2016 12:34 PM, "MickaÃl SalaÃn" <mic@xxxxxxxxxxx> wrote:
>>>
>>> Add LSM hooks which can be used by userland through Landlock (eBPF)
>>> programs. This programs are limited to a whitelist of functions (cf.
>>> next commit). The eBPF program context is depicted by the struct
>>> landlock_data (cf. include/uapi/linux/bpf.h):
>>> * hook: LSM hook ID (useful when using the same program for multiple LSM
>>>   hooks);
>>> * cookie: the 16-bit value from the seccomp filter that triggered this
>>>   Landlock program;
>>> * args[6]: array of LSM hook arguments.
>>>
>>> The LSM hook arguments can contain raw values as integers or
>>> (unleakable) pointers. The only way to use the pointers are to pass them
>>> to an eBPF function according to their types (e.g. the
>>> bpf_landlock_cmp_fs_beneath_with_struct_file function can use a struct
>>> file pointer).
>>>
>>> For now, there is three hooks for file system access control:
>>> * file_open;
>>> * file_permission;
>>> * mmap_file.
>>>
>>
>> What's the purpose of exposing struct cred * to userspace?  It's
>> primarily just an optimization to save a bit of RAM, and it's a
>> dubious optimization at that.  What are you using it for?  Would it
>> make more sense to use struct task_struct * or struct pid * instead?
>>
>> Also, exposing struct cred * has a really weird side-effect: it allows
>> (maybe even encourages) checking for pointer equality between two
>> struct cred * objects.  Doing so will have erratic results.
>>
>
> The pointers exposed in the ePBF context are not directly readable by an
> unprivileged eBPF program thanks to the strong typing of the Landlock
> context and the static eBPF verification. There is no way to leak a
> kernel pointer to userspace from an unprivileged eBPF program: pointer
> arithmetic and comparison are prohibited. Pointers can only be pass as
> argument to dedicated eBPF functions.

I'm not talking about leaking the value -- I'm talking about leaking
the predicate (a == b) for two struct cred pointers.  That predicate
shouldn't be available because it has very odd effects.

>
> For now, struct cred * is simply not used by any eBPF function and then
> not usable at all. It only exist here because I map the LSM hook
> arguments in a generic/automatic way to the eBPF context.

Maybe remove it from this patch set then?

--Andy