linux-kernel - Re: [PATCH v2 5/6] LSM: secctx provider check on release

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Message-ID: <5b6addd938c9feae0b4df8f54d56f9f0@paul-moore.com>
Date: Mon, 21 Oct 2024 19:39:50 -0400
From: Paul Moore <paul@...l-moore.com>
To: Casey Schaufler <casey@...aufler-ca.com>, casey@...aufler-ca.com, linux-security-module@...r.kernel.org
Cc: jmorris@...ei.org, serge@...lyn.com, keescook@...omium.org, john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp, stephen.smalley.work@...il.com, linux-kernel@...r.kernel.org, selinux@...r.kernel.org, mic@...ikod.net
Subject: Re: [PATCH v2 5/6] LSM: secctx provider check on release

On Oct 14, 2024 Casey Schaufler <casey@...aufler-ca.com> wrote:
> 
> Verify that the LSM releasing the secctx is the LSM that
> allocated it. This was not necessary when only one LSM could
> create a secctx, but once there can be more than one it is.
> 
> Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
> ---
>  security/apparmor/secid.c | 10 ++--------
>  security/selinux/hooks.c  | 10 ++--------
>  2 files changed, 4 insertions(+), 16 deletions(-)
> 
> diff --git a/security/apparmor/secid.c b/security/apparmor/secid.c
> index 5d92fc3ab8b4..974f802cbe5a 100644
> --- a/security/apparmor/secid.c
> +++ b/security/apparmor/secid.c
> @@ -122,14 +122,8 @@ int apparmor_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
>  
>  void apparmor_release_secctx(struct lsm_context *cp)
>  {
> -	/*
> -	 * stacking scaffolding:
> -	 * When it is possible for more than one LSM to provide a
> -	 * release hook, do this check:
> -	 * if (cp->id == LSM_ID_APPARMOR || cp->id == LSM_ID_UNDEF)
> -	 */
> -
> -	kfree(cp->context);
> +	if (cp->id == LSM_ID_APPARMOR)
> +		kfree(cp->context);

Should we set cp->context to NULL too?  One could argue that it's an
unecessary assignment, given the cp->id checks, and they wouldn't be
wrong, but considering the potential for a BPF LSM to do things with
a lsm_context, I wonder if resetting the pointer to NULL is the
smart thing to do.

This obviously applies to the SELinux code (below) too.

>  }
>  
>  /**
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 79776a5e651d..b9286c2c5efe 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6640,14 +6640,8 @@ static int selinux_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
>  
>  static void selinux_release_secctx(struct lsm_context *cp)
>  {
> -	/*
> -	 * stacking scaffolding:
> -	 * When it is possible for more than one LSM to provide a
> -	 * release hook, do this check:
> -	 * if (cp->id == LSM_ID_SELINUX || cp->id == LSM_ID_UNDEF)
> -	 */
> -
> -	kfree(cp->context);
> +	if (cp->id == LSM_ID_SELINUX)
> +		kfree(cp->context);
>  }
>  
>  static void selinux_inode_invalidate_secctx(struct inode *inode)

--
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