BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM]

[Daniel Borkmann]

On 2/12/20 12:26 AM, Alexei Starovoitov wrote:
> On Tue, Feb 11, 2020 at 1:38 PM Alexei Starovoitov
> <alexei.starovoitov at gmail.com> wrote:
>>
>> On Tue, Feb 11, 2020 at 09:33:49PM +0100, Jann Horn wrote:
>>>>
>>>> Got it. Then let's whitelist them ?
>>>> All error injection points are marked with ALLOW_ERROR_INJECTION().
>>>> We can do something similar here, but let's do it via BTF and avoid
>>>> abusing yet another elf section for this mark.
>>>> I think BTF_TYPE_EMIT() should work. Just need to pick explicit enough
>>>> name and extensive comment about what is going on.
>>>
>>> Sounds reasonable to me. :)
>>
>> awesome :)
> 
> Looks like the kernel already provides this whitelisting.
> $ bpftool btf dump file /sys/kernel/btf/vmlinux |grep FUNC|grep '\<security_'
> gives the list of all LSM hooks that lsm-bpf will be able to attach to.
> There are two exceptions there security_add_hooks() and security_init().
> Both are '__init'. Too late for lsm-bpf to touch.
> So filtering BTF funcs by 'security_' prefix will be enough.
> It should be documented though.
> The number of attachable funcs depends on kconfig which is
> a nice property and further strengthen the point that
> lsm-bpf is very much kernel specific.
> We probably should blacklist security_bpf*() hooks though.

One thing that is not quite clear to me wrt the fexit approach; assuming
we'd whitelist something like security_inode_link():

int security_inode_link(struct dentry *old_dentry, struct inode *dir,
                          struct dentry *new_dentry)
{
         if (unlikely(IS_PRIVATE(d_backing_inode(old_dentry))))
                 return 0;
         return call_int_hook(inode_link, 0, old_dentry, dir, new_dentry);
}

Would this then mean the BPF prog needs to reimplement above check by
probing old_dentry->d_inode to later ensure its verdict stays 0 there
too, or that such extra code is to be moved to call-sites instead? If
former, what about more complex logic?

Another approach could be to have a special nop inside call_int_hook()
macro which would then get patched to avoid these situations. Somewhat
similar like static keys where it could be defined anywhere in text but
with updating of call_int_hook()'s RC for the verdict.

Thanks,
Daniel