Subject: Re: [PATCH net-next v2] bridge: set is_local and is_static before fdb entry is added to the fdb hashtable

From: Nikolay Aleksandrov <[email-masked]>
Date: Tue, 27 Oct 2015 14:31:48 +0100

On 10/27/2015 01:12 PM, Roopa Prabhu wrote:
> From: Roopa Prabhu <[email-masked]>
> 
> Problem Description:
> We can add fdbs pointing to the bridge with NULL ->dst but that has a
> few race conditions because br_fdb_insert() is used which first creates
> the fdb and then, after the fdb has been published/linked, sets
> "is_local" to 1 and in that time frame if a packet arrives for that fdb
> it may see it as non-local and either do a NULL ptr dereference in
> br_forward() or attach the fdb to the port where it arrived, and later
> br_fdb_insert() will make it local thus getting a wrong fdb entry.
> Call chain br_handle_frame_finish() -> br_forward():
> But in br_handle_frame_finish() in order to call br_forward() the dst
> should not be local i.e. skb != NULL, whenever the dst is
> found to be local skb is set to NULL so we can't forward it,
> and here comes the problem since it's running only
> with RCU when forwarding packets it can see the entry before "is_local"
> is set to 1 and actually try to dereference NULL.
> The main issue is that if someone sends a packet to the switch while
> it's adding the entry which points to the bridge device, it may
> dereference NULL ptr. This is needed now after we can add fdbs
> pointing to the bridge.  This poses a problem for
> br_fdb_update() as well, while someone's adding a bridge fdb, but
> before it has is_local == 1, it might get moved to a port if it comes
> as a source mac and then it may get its "is_local" set to 1
> 
> This patch changes fdb_create to take is_local and is_static as
> arguments to set these values in the fdb entry before it is added to the
> hash. Also adds null check for port in br_forward.
> 
> Reported-by: Nikolay Aleksandrov <[email-masked]>
> Signed-off-by: Roopa Prabhu <[email-masked]>
> ---
> v2 - fix compile error reported by kbuild test robot.
>      Accidently i had posted an older version of the patch
> 

Thanks for fixing this,

Fixes: 3741873b4f73 ("bridge: allow adding of fdb entries pointing to the bridge device")
Reviewed-by: Nikolay Aleksandrov <[email-masked]>