From: Casey Schaufler <casey@schaufler-ca.com>
To: KP Singh <kpsingh@kernel.org>,
Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Roberto Sassu <roberto.sassu@huaweicloud.com>,
Florent Revest <revest@chromium.org>,
Brendan Jackman <jackmanb@chromium.org>,
Paul Moore <paul@paul-moore.com>,
James Morris <jmorris@namei.org>,
"Serge E . Hallyn" <serge@hallyn.com>, bpf <bpf@vger.kernel.org>,
LSM List <linux-security-module@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
nicolas.bouchinet@clip-os.org, Mimi Zohar <zohar@linux.ibm.com>,
linux-integrity <linux-integrity@vger.kernel.org>,
casey@schaufler-ca.com
Subject: Re: [RFC][PATCH] bpf: Check xattr name/value pair from bpf_lsm_inode_init_security()
Date: Thu, 27 Oct 2022 08:52:50 -0700 [thread overview]
Message-ID: <fc303279-5853-4be8-6055-649f23a3ac7a@schaufler-ca.com> (raw)
In-Reply-To: <CACYkzJ4ak4=qPNQxVckvn3c8CZpXkXSLSyYa_HCU-RJNyuLoZg@mail.gmail.com>
On 10/27/2022 3:39 AM, KP Singh wrote:
> On Wed, Oct 26, 2022 at 7:14 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>> On Wed, Oct 26, 2022 at 1:42 AM Roberto Sassu
>> <roberto.sassu@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@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.
It might be difficult to identify which hooks will never be useful
in a general purpose programming system like BPF. I do suggest that,
if at all possible, you restrict any hook that uses or provides a
secid. That will take out the bulk of the "dangerous" hooks.
> 2. A verifier function that checks return values of LSM
> hooks.
That would be grand.
> 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.
The exceptions are pretty rare, and I don't see a reason why
we couldn't "normalize", or at least more clearly document the
outliers.
next prev parent reply other threads:[~2022-10-27 15:53 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-21 16:46 [RFC][PATCH] bpf: Check xattr name/value pair from bpf_lsm_inode_init_security() Roberto Sassu
2022-10-23 23:36 ` Alexei Starovoitov
2022-10-24 9:25 ` Roberto Sassu
2022-10-24 15:28 ` Roberto Sassu
2022-10-25 2:13 ` Alexei Starovoitov
2022-10-25 7:43 ` Roberto Sassu
2022-10-25 14:57 ` Casey Schaufler
2022-10-26 6:37 ` Alexei Starovoitov
2022-10-26 8:42 ` Roberto Sassu
2022-10-26 17:14 ` Alexei Starovoitov
2022-10-27 10:39 ` KP Singh
2022-10-27 15:52 ` Casey Schaufler [this message]
2022-10-28 8:48 ` Roberto Sassu
2022-10-28 15:01 ` Casey Schaufler
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=fc303279-5853-4be8-6055-649f23a3ac7a@schaufler-ca.com \
--to=casey@schaufler-ca.com \
--cc=alexei.starovoitov@gmail.com \
--cc=bpf@vger.kernel.org \
--cc=jackmanb@chromium.org \
--cc=jmorris@namei.org \
--cc=kpsingh@kernel.org \
--cc=linux-integrity@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-security-module@vger.kernel.org \
--cc=nicolas.bouchinet@clip-os.org \
--cc=paul@paul-moore.com \
--cc=revest@chromium.org \
--cc=roberto.sassu@huaweicloud.com \
--cc=serge@hallyn.com \
--cc=zohar@linux.ibm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).