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

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Message-ID: <617a2679-404c-4127-8dfd-4f3895e2372f@schaufler-ca.com>
Date: Mon, 21 Oct 2024 17:05:59 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: Paul Moore <paul@...l-moore.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,
 Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v2 5/6] LSM: secctx provider check on release

On 10/21/2024 4:39 PM, Paul Moore wrote:
> 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.

Wouldn't hurt. I'll go ahead and add that. If a BPF LSM does anything
with a lsm_context we're likely to hear about the many issues quite
quickly.

>
> 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)
> --
> paul-moore.com
>