[RFC][PATCH] bpf: Check xattr name/value pair from bpf_lsm_inode_init_security()

[Casey Schaufler]

On 10/28/2022 1:48 AM, Roberto Sassu wrote:
> On Thu, 2022-10-27 at 12:39 +0200, KP Singh wrote:
>> On Wed, Oct 26, 2022 at 7:14 PM Alexei Starovoitov
>> <alexei.starovoitov at gmail.com> wrote:
>>> On Wed, Oct 26, 2022 at 1:42 AM Roberto Sassu
>>> <roberto.sassu at huaweicloud.com> wrote:
>>>> On 10/26/2022 8:37 AM, Alexei Starovoitov wrote:
>>>>> On Tue, Oct 25, 2022 at 7:58 AM Casey Schaufler <
>>>>> casey at schaufler-ca.com> wrote:
>>>>>> On 10/25/2022 12:43 AM, Roberto Sassu wrote:
>>>>>>> On Mon, 2022-10-24 at 19:13 -0700, Alexei Starovoitov
>>>>>>> wrote:
>>>>>>>> I'm looking at security_inode_init_security() and it is
>>>>>>>> indeed messy.
>>>>>>>> Per file system initxattrs callback that processes
>>>>>>>> kmalloc-ed
>>>>>>>> strings.
>>>>>>>> Yikes.
>>>>>>>>
>>>>>>>> In the short term we should denylist inode_init_security
>>>>>>>> hook to
>>>>>>>> disallow attaching bpf-lsm there. set/getxattr should be
>>>>>>>> done
>>>>>>>> through kfuncs instead of such kmalloc-a-string hack.
>>>>>>> Inode_init_security is an example. It could be that the
>>>>>>> other hooks are
>>>>>>> affected too. What happens if they get arbitrary positive
>>>>>>> values too?
>>>>>> TL;DR - Things will go cattywampus.
>>>>>>
>>>>>> The LSM infrastructure is an interface that has "grown
>>>>>> organically",
>>>>>> and isn't necessarily consistent in what it requires of the
>>>>>> security
>>>>>> module implementations. There are cases where it assumes that
>>>>>> the
>>>>>> security module hooks are well behaved, as you've discovered.
>>>>>> I have
>>>>>> no small amount of fear that someone is going to provide an
>>>>>> eBPF
>>>>>> program for security_secid_to_secctx(). There has been an
>>>>>> assumption,
>>>>>> oft stated, that all security modules are going to be
>>>>>> reviewed as
>>>>>> part of the upstream process. The review process ought to
>>>>>> catch hooks
>>>>>> that return unacceptable values. Alas, we've lost that with
>>>>>> BPF.
>>>>>>
>>>>>> It would take a(nother) major overhaul of the LSM
>>>>>> infrastructure to
>>>>>> make it safe against hooks that are not well behaved. From
>>>>>> what I have
>>>>>> seen so far it wouldn't be easy/convenient/performant to do
>>>>>> it in the
>>>>>> BPF security module either. I personally think that BPF needs
>>>>>> to
>>>>>> ensure that the eBPF implementations don't return
>>>>>> inappropriate values,
>>>>>> but I understand why that is problematic.
>>>>> That's an accurate statement. Thank you.
>>>>>
>>>>> Going back to the original question...
>>>>> We fix bugs when we discover them.
>>>>> Regardless of the subsystem they belong to.
>>>>> No finger pointing.
>>>> I'm concerned about the following situation:
>>>>
>>>> struct <something> *function()
>>>> {
>>>>
>>>>         ret = security_*();
>>>>         if (ret)
>>>>                 return ERR_PTR(ret);
>>>>
>>>> }
>>>>
>>>> int caller()
>>>> {
>>>>         ptr = function()
>>>>         if (IS_ERR(ptr)
>>>>                 goto out;
>>>>
>>>>         <use of invalid pointer>
>>>> }
>>>>
>>>> I quickly found an occurrence of this:
>>>>
>>>> static int lookup_one_common()
>>>> {
>>>>
>>>> [...]
>>>>
>>>>         return inode_permission();
>>>> }
>>>>
>>>> struct dentry *try_lookup_one_len()
>>>> {
>>>>
>>>> [...]
>>>>
>>>>          err = lookup_one_common(&init_user_ns, name, base, len,
>>>> &this);
>>>>          if (err)
>>>>                  return ERR_PTR(err);
>>>>
>>>>
>>>> Unfortunately, attaching to inode_permission causes the kernel
>>>> to crash immediately (it does not happen with negative return
>>>> values).
>>>>
>>>> So, I think the fix should be broader, and not limited to the
>>>> inode_init_security hook. Will try to see how it can be fixed.
>>> I see. Let's restrict bpf-lsm return values to IS_ERR_VALUE.
>>> Trivial verifier change.
>> Thanks, yes this indeed is an issue. We need to do a few things:
>>
>> 1. Restrict some hooks that we know the BPF LSM will never need.
>> 2. A verifier function that checks return values of LSM
>> hooks.
>> For most LSK hooks IS_ERR_VALUE is fine, however, there are some
>> hooks
>> like *xattr hooks that use a return value of 1 to indicate a
>> capability check is required which might need special handling.
> I looked at security.c:
>
> /*
>  * SELinux and Smack integrate the cap call,
>  * so assume that all LSMs supplying this call do so.
>  */
>
> Other than checking the return value, probably we should also wrap
> bpf_lsm_inode_{set,remove}xattr() to do the capability check, right?

Long term I hope to fix the way capabilities are dealt with in
this hook, but for now your suggestion seems reasonable. 

>
> Roberto
>