linux-kernel - Re: [RFC][PATCH] bpf: Check xattr name/value pair from bpf_lsm_inode_init

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Date:   Tue, 25 Oct 2022 07:57:54 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     KP Singh <kpsingh@...nel.org>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>, bpf <bpf@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        nicolas.bouchinet@...p-os.org, Mimi Zohar <zohar@...ux.ibm.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        casey@...aufler-ca.com
Subject: Re: [RFC][PATCH] bpf: Check xattr name/value pair from
 bpf_lsm_inode_init_security()

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.

> Thanks
>
> Roberto
>