linux-kernel - Re: [PATCH v3 2/3] lsm: introduce security_lsm_config_*

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Message-ID: <945bf443-32b4-4432-8702-41ff7b15e420@I-love.SAKURA.ne.jp>
Date: Wed, 25 Jun 2025 10:08:55 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Maxime Bélair <maxime.belair@...onical.com>,
        linux-security-module@...r.kernel.org
Cc: 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, song@...nel.org, rdunlap@...raread.org,
        linux-api@...r.kernel.org, apparmor@...ts.ubuntu.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] lsm: introduce security_lsm_config_*_policy hooks

On 2025/06/24 23:30, Maxime Bélair wrote:
> +config LSM_CONFIG_SELF_POLICY_MAX_BUFFER_SIZE
> +	int "Maximum buffer size for lsm_config_self_policy"
> +	range 16384 1073741824
> +	depends on SECURITY
> +	default 4194304
> +	help
> +	  The maximum size of the buffer argument of lsm_config_self_policy.
> +
> +	  The default value of 4194304 (4MiB) is reasonable and should be large
> +	  enough to fit policies in for most cases.
> +

Do we want to define LSM_CONFIG_{SELF,SYSTEM}_POLICY_MAX_BUFFER_SIZE as Kconfig?

If security_lsm_config_{self,system}_policy() are meant to be used by multiple
LSM modules, the upper limit each LSM module wants to impose would vary. Also,
1073741824 is larger than KMALLOC_MAX_SIZE; kmalloc()-based memory copying
functions will hit WARN_ON_ONCE_GFP() at __alloc_frozen_pages_noprof().

Since some of LSM modules might use vmalloc()-based memory copying functions from
security_lsm_config_{self,system}_policy(), the upper limit should be imposed by
individual LSM module which provides security_lsm_config_{self,system}_policy().