Looks like issue in handling active_nodes count in 4.19 kernel .

[rsiddoji at codeaurora.org]

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 . 

> /*@ 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?