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

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Message-ID: <bc252425-2703-48c4-a1fa-9268124c2386@canonical.com>
Date: Wed, 7 May 2025 17:37:25 +0200
From: Maxime Bélair <maxime.belair@...onical.com>
To: Song Liu <song@...nel.org>
Cc: linux-security-module@...r.kernel.org, john.johansen@...onical.com,
 paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com, mic@...ikod.net,
 kees@...nel.org, stephen.smalley.work@...il.com, casey@...aufler-ca.com,
 takedakn@...data.co.jp, penguin-kernel@...ove.sakura.ne.jp,
 linux-api@...r.kernel.org, apparmor@...ts.ubuntu.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] lsm: introduce security_lsm_manage_policy hook



On 5/7/25 08:19, Song Liu wrote:
> On Tue, May 6, 2025 at 7:40 AM Maxime Bélair
> <maxime.belair@...onical.com> wrote:
>>
>> Define a new LSM hook security_lsm_manage_policy and wire it into the
>> lsm_manage_policy() syscall so that LSMs can register a unified interface
>> for policy management. This initial, minimal implementation only supports
>> the LSM_POLICY_LOAD operation to limit changes.
>>
>> Signed-off-by: Maxime Bélair <maxime.belair@...onical.com>
> [...]
>> diff --git a/security/security.c b/security/security.c
>> index fb57e8fddd91..256104e338b1 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -5883,6 +5883,27 @@ int security_bdev_setintegrity(struct block_device *bdev,
>>  }
>>  EXPORT_SYMBOL(security_bdev_setintegrity);
>>
>> +/**
>> + * security_lsm_manage_policy() - Manage the policies of LSMs
>> + * @lsm_id: id of the lsm to target
>> + * @op: Operation to perform (one of the LSM_POLICY_XXX values)
>> + * @buf:  userspace pointer to policy data
>> + * @size: size of @buf
>> + * @flags: lsm policy management flags
>> + *
>> + * Manage the policies of a LSM. This notably allows to update them even when
>> + * the lsmfs is unavailable is restricted. Currently, only LSM_POLICY_LOAD is
>> + * supported.
>> + *
>> + * Return: Returns 0 on success, error on failure.
>> + */
>> +int security_lsm_manage_policy(u32 lsm_id, u32 op, void __user *buf,
>> +                              size_t size, u32 flags)
>> +{
>> +       return call_int_hook(lsm_manage_policy, lsm_id, op, buf, size, flags);
> 
> If the LSM doesn't implement this hook, sys_lsm_manage_policy will return 0
> for any inputs, right? This is gonna be so confusing for users.

Indeed, that was an oversight. It will return -EOPNOTSUPP in the next patch revision.

> 
> Thanks,
> Song

Thanks,

Maxime