[PATCH v7 22/28] SELinux: Verify LSM display sanity in binder

[Kees Cook]

On Wed, Aug 07, 2019 at 12:44:04PM -0700, Casey Schaufler wrote:
> Verify that the tasks on the ends of a binder transaction
> use LSM display values that don't cause SELinux contexts
> to be interpreted by another LSM or another LSM's context
> to be interpreted by SELinux. No judgement is made in cases
> that where SELinux contexts are not used in the binder
> transaction.
> 
> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
> ---
>  security/selinux/hooks.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 352be16a887d..fcad2e3432d2 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2009,6 +2009,28 @@ static inline u32 open_file_to_av(struct file *file)
>  	return av;
>  }
>  
> +/*
> + * Verify that if the "display" LSM is SELinux for either task
> + * that it is for both tasks.
> + */
> +static inline bool compatible_task_displays(struct task_struct *here,
> +					    struct task_struct *there)
> +{
> +	int h = lsm_task_display(here);
> +	int t = lsm_task_display(there);
> +
> +	if (h == t)
> +		return true;
> +
> +	/* unspecified is only ok if SELinux isn't going to be involved */
> +	if (selinux_lsmid.slot == 0)
> +		return ((h == 0 && t == LSMBLOB_INVALID) ||
> +			(t == 0 && h == LSMBLOB_INVALID));

What is "0" here? Doesn't that just mean the first LSM. I though only -1
had a special meaning (and had a #define name for it).

-Kees

> +
> +	/* it's ok only if neither display is SELinux */
> +	return (h != selinux_lsmid.slot && t != selinux_lsmid.slot);
> +}
> +
>  /* Hook functions begin here. */
>  
>  static int selinux_binder_set_context_mgr(struct task_struct *mgr)
> @@ -2016,6 +2038,9 @@ static int selinux_binder_set_context_mgr(struct task_struct *mgr)
>  	u32 mysid = current_sid();
>  	u32 mgrsid = task_sid(mgr);
>  
> +	if (!compatible_task_displays(current, mgr))
> +		return -EINVAL;
> +
>  	return avc_has_perm(&selinux_state,
>  			    mysid, mgrsid, SECCLASS_BINDER,
>  			    BINDER__SET_CONTEXT_MGR, NULL);
> @@ -2029,6 +2054,9 @@ static int selinux_binder_transaction(struct task_struct *from,
>  	u32 tosid = task_sid(to);
>  	int rc;
>  
> +	if (!compatible_task_displays(from, to))
> +		return -EINVAL;
> +
>  	if (mysid != fromsid) {
>  		rc = avc_has_perm(&selinux_state,
>  				  mysid, fromsid, SECCLASS_BINDER,
> @@ -2048,6 +2076,9 @@ static int selinux_binder_transfer_binder(struct task_struct *from,
>  	u32 fromsid = task_sid(from);
>  	u32 tosid = task_sid(to);
>  
> +	if (!compatible_task_displays(from, to))
> +		return -EINVAL;
> +
>  	return avc_has_perm(&selinux_state,
>  			    fromsid, tosid, SECCLASS_BINDER, BINDER__TRANSFER,
>  			    NULL);
> @@ -2064,6 +2095,9 @@ static int selinux_binder_transfer_file(struct task_struct *from,
>  	struct common_audit_data ad;
>  	int rc;
>  
> +	if (!compatible_task_displays(from, to))
> +		return -EINVAL;
> +
>  	ad.type = LSM_AUDIT_DATA_PATH;
>  	ad.u.path = file->f_path;
>  
> -- 
> 2.20.1
> 

-- 
Kees Cook