linux-kernel - Re: [PATCH 2/3] lsm: introduce security_lsm_manage

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Message-ID: <efe5b15a-6141-424a-8391-9092e79e4acf@schaufler-ca.com>
Date: Thu, 8 May 2025 09:54:19 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: John Johansen <john.johansen@...onical.com>,
 Paul Moore <paul@...l-moore.com>, Maxime Bélair
 <maxime.belair@...onical.com>
Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
 linux-security-module@...r.kernel.org, jmorris@...ei.org, serge@...lyn.com,
 mic@...ikod.net, kees@...nel.org, stephen.smalley.work@...il.com,
 takedakn@...data.co.jp, linux-api@...r.kernel.org,
 apparmor@...ts.ubuntu.com, linux-kernel@...r.kernel.org,
 Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 2/3] lsm: introduce security_lsm_manage_policy hook

On 5/8/2025 1:29 AM, John Johansen wrote:
> On 5/7/25 13:25, Paul Moore wrote:
>> On Wed, May 7, 2025 at 6:41 AM Tetsuo Handa
>> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>> On 2025/05/06 23:32, Maxime Bélair wrote:
>>>> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
>>>> index dcaad8818679..b39e6635a7d5 100644
>>>> --- a/security/lsm_syscalls.c
>>>> +++ b/security/lsm_syscalls.c
>>>> @@ -122,5 +122,10 @@ SYSCALL_DEFINE3(lsm_list_modules, u64 __user
>>>> *, ids, u32 __user *, size,
>>>>   SYSCALL_DEFINE5(lsm_manage_policy, u32, lsm_id, u32, op, void
>>>> __user *, buf, u32
>>>>                __user *, size, u32, flags)
>>>>   {
>>>> -     return 0;
>>>> +     size_t usize;
>>>> +
>>>> +     if (get_user(usize, size))
>>>> +             return -EFAULT;
>>>> +
>>>> +     return security_lsm_manage_policy(lsm_id, op, buf, usize,
>>>> flags);
>>>>   }
>>>
>>> syzbot will report user-controlled unbounded huge size memory
>>> allocation attempt. ;-)
>>>
>>> This interface might be fine for AppArmor, but TOMOYO won't use this
>>> interface because
>>> TOMOYO's policy is line-oriented ASCII text data where the
>>> destination is switched via
>>> pseudo‑filesystem's filename ...
>>
>> While Tetsuo's comment is limited to TOMOYO, I believe the argument
>> applies to a number of other LSMs as well.  The reality is that there
>> is no one policy ideal shared across LSMs and that complicates things
>> like the lsm_manage_policy() proposal.  I'm intentionally saying
>> "complicates" and not "prevents" because I don't want to flat out
>> reject something like this, but I think there needs to be a larger
>> discussion among the different LSM groups about what such an API
>> should look like.  We may not need to get every LSM to support this
>> new API, but we need to get something that would work for a
>> significant majority and would be general/extensible enough that we
>> would expect it to work with the majority of future LSMs (as much as
>> we can predict the future anyway).
>>
>
> yep, I look at this is just a starting point for discussion. There
> isn't going to be any discussion without some code, so here is a v1
> that supports a single LSM let the bike shedding begin.

Aside from the issues with allocating a buffer for a big policy
I don't see a problem with this proposal. The system call looks
a lot like the other LSM interfaces, so any developer who likes
those ought to like this one. The infrastructure can easily check
the lsm_id and only call the appropriate LSM hook, so no one
is going to be interfering with other modules.