Re: [PATCH RFC v2 5/5] SELinux: Support SELinux determination of side-channel vulnerability

[Stephen Smalley <sds@tycho.nsa.gov>]

On 08/20/2018 12:59 PM, Schaufler, Casey wrote:
>> -----Original Message-----
>> From: Stephen Smalley [mailto:sds@tycho.nsa.gov]
>> Sent: Monday, August 20, 2018 9:03 AM
>> To: Schaufler, Casey <casey.schaufler@intel.com>; kernel-
>> hardening@lists.openwall.com; linux-kernel@vger.kernel.org; linux-security-
>> module@vger.kernel.org; selinux@tycho.nsa.gov; Hansen, Dave
>> <dave.hansen@intel.com>; Dock, Deneen T <deneen.t.dock@intel.com>;
>> kristen@linux.intel.com; arjan@linux.intel.com
>> Subject: Re: [PATCH RFC v2 5/5] SELinux: Support SELinux determination of
>> side-channel vulnerability
>>
>> On 08/17/2018 06:16 PM, Casey Schaufler wrote:
>>> SELinux considers tasks to be side-channel safe if they
>>> have PROCESS_SHARE access.
>>
>> Now the description and the code no longer match.
> 
> You're right.
> 
>>>
>>> Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
>>> ---
>>>    security/selinux/hooks.c | 9 +++++++++
>>>    1 file changed, 9 insertions(+)
>>>
>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>> index a8bf324130f5..7fbd7d7ac1cb 100644
>>> --- a/security/selinux/hooks.c
>>> +++ b/security/selinux/hooks.c
>>> @@ -4219,6 +4219,14 @@ static void selinux_task_to_inode(struct
>> task_struct *p,
>>>    	spin_unlock(&isec->lock);
>>>    }
>>>
>>> +static int selinux_task_safe_sidechannel(struct task_struct *p)
>>> +{
>>> +	struct av_decision avd;
>>> +
>>> +	return avc_has_perm_noaudit(&selinux_state, current_sid(),
>> task_sid(p),
>>> +				    SECCLASS_FILE, FILE__READ, 0, &avd);
>>> +}
>>
>> And my question from before still stands:  why do we need a new hook and
>> new security module instead of just using ptrace_may_access()?
> 
> Locking. The SELinux check, for example, will lock up solid while trying
> to generate an audit record. There is no good reason aside from coding
> convenience to assume that the same restrictions will apply for side-channel
> as apply to ptrace. I'm actually a touch surprised you're not suggesting a
> separate SECCLASS or access mode for the SELinux hook.

The PTRACE_MODE_NOAUDIT flag to ptrace_may_access() would address the 
locking concern. Duplicating the ptrace access checking logic seems 
prone to errors and inconsistencies. I can't imagine policy writers 
understanding what "safe sidechannel" means, much less deciding when to 
allow it.

> 
>>
>>> +
>>>    /* Returns error only if unable to parse addresses */
>>>    static int selinux_parse_skb_ipv4(struct sk_buff *skb,
>>>    			struct common_audit_data *ad, u8 *proto)
>>> @@ -7002,6 +7010,7 @@ static struct security_hook_list selinux_hooks[]
>> __lsm_ro_after_init = {
>>>    	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
>>>    	LSM_HOOK_INIT(task_kill, selinux_task_kill),
>>>    	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
>>> +	LSM_HOOK_INIT(task_safe_sidechannel,
>> selinux_task_safe_sidechannel),
>>>
>>>    	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
>>>    	LSM_HOOK_INIT(ipc_getsecid, selinux_ipc_getsecid),
>>>
>