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

[Casey Schaufler <casey@schaufler-ca.com>]

On 8/8/2019 2:55 PM, Kees Cook wrote:
> 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@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).

I try not to write obscure code, but I seem to have done so here.

The lsm in slot 0 (the first registered "display" lsm) will
get used if the display value is LSMBLOB_INVALID. We've already
checked to see if the display values are the same, and they're not.

If selinux is in slot 0, one of the display values is 0 and the
other is LSMBLOB_INVALID, the displays are compatible. Otherwise,
they're not. If selinux is not in slot 0 and either of the displays
slots is selinux's slot, it's not compatible.

Simple, no?

I'll have a go at making the code more obvious or, failing
that, better documented.

>
> -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
>>