Looks like issue in handling active_nodes count in 4.19 kernel .

[Stephen Smalley]

On 12/11/19 9:37 AM, Stephen Smalley wrote:
> On 12/9/19 1:30 PM, rsiddoji at codeaurora.org wrote:
>> Thanks for quick response , yes it will be helpful if you can raise 
>> the change .
>> On the second issue  in  avc_alloc_node we are trying to check the  
>> slot status  as    active_nodes  > 512 ( default )
>> Where  checking the occupancy  should be corrected as     active_nodes 
>> > 80% of slots occupied  or 16*512 or
>> May be we need to use a different logic .
> 
> Are you seeing an actual problem with this in practice, and if so, what 
> exactly is it that you are seeing and do you have a reproducer?

BTW, on Linux distributions, there is an avcstat(8) utility that can be 
used to monitor the AVC statistics, or you can directly read the stats 
from the kernel via /sys/fs/selinux/avc/cache_stats

> 
>>
>>> /*@ static struct avc_node *avc_alloc_node(struct selinux_avc *avc) */
>>>
>>>        if (atomic_inc_return(&avc->avc_cache.active_nodes) >
>>>            avc->avc_cache_threshold)      //  default  threshold is 512
>>>            avc_reclaim_node(avc);
>>>
>>
>> Regards,
>> Ravi
>>
>> -----Original Message-----
>> From: selinux-owner at vger.kernel.org <selinux-owner at vger.kernel.org> On 
>> Behalf Of Stephen Smalley
>> Sent: Monday, December 9, 2019 11:35 PM
>> To: rsiddoji at codeaurora.org; selinux at vger.kernel.org
>> Cc: paul at paul-moore.com; linux-security-module at vger.kernel.org
>> Subject: Re: Looks like issue in handling active_nodes count in 4.19 
>> kernel .
>>
>> On 12/9/19 10:55 AM, rsiddoji at codeaurora.org wrote:
>>> Hi team ,
>>> Looks like we have  issue in handling the  "active_nodes" count in the
>>> Selinux - avc.c file.
>>> Where  avc_cache.active_nodes increase more than slot array   and code
>>> frequency calling of avc_reclaim_node()  from  avc_alloc_node() ;
>>>
>>> Where following are the 2 instance which seem to  possible culprits
>>> which are seen on 4.19 kernel . Can you  comment if my understand is 
>>> wrong.
>>>
>>>
>>> #1. if we see the  active_nodes count is incremented in
>>> avc_alloc_node
>>> (avc) which is called in avc_insert()
>>> Where if the code take  failure path on  avc_xperms_populate  the code
>>> will not decrement this counter .
>>>
>>>
>>> static struct avc_node *avc_insert(struct selinux_avc *avc,
>>>                    u32 ssid, u32 tsid, u16 tclass,
>>>                       struct av_decision *avd,
>>> ....
>>>     node = avc_alloc_node(avc);  //incremented here ....
>>>                 rc = avc_xperms_populate(node, xp_node);  //
>>> possibilities of this getting failure is there .
>>>         if (rc) {
>>>             kmem_cache_free(avc_node_cachep, node);  // but on 
>>> failure we are
>>> not decrementing active_nodes ?
>>>             return NULL;
>>>            }
>>
>> I think you are correct; we should perhaps be calling avc_node_kill() 
>> here as we do in an earlier error path?
>>
>>>
>>> #2.  where it looks like the logic on comparing the  active_nodes
>>> against avc_cache_threshold seems  wired  as the count of active nodes
>>> is always going to be
>>>    more than 512 will may land in simply  removing /calling
>>> avc_reclaim_node frequently much before the slots are full maybe we
>>> are not using cache at best ?
>>>    we should be comparing with some high watermark ? or my
>>> understanding wrong ?
>>> /*@ static struct avc_node *avc_alloc_node(struct selinux_avc *avc) */
>>>
>>>        if (atomic_inc_return(&avc->avc_cache.active_nodes) >
>>>            avc->avc_cache_threshold)      //  default  threshold is 512
>>>            avc_reclaim_node(avc);
>>>
>>
>> Not entirely sure what you are asking here.  avc_reclaim_node() should 
>> reclaim multiple nodes up to AVC_CACHE_RECLAIM.  Possibly that should 
>> be configurable via selinuxfs too, and/or calculated from 
>> avc_cache_threshold in some way?
>>
>> Were you interested in creating a patch to fix the first issue above 
>> or looking to us to do so?
>>
>>
>>
>