[PATCH] net: xfrm: fix a race condition during allocing spi

[zhuoliang.zhang]

hi maintainer:

there are 2 related hash lists : net->xfrm.state_bydst and
net->xfrm.state_byspi:

1. a new state x is alloced in xfrm_state_alloc() and added into the
bydst hlist in  __find_acq_core() on the LHS;
2. on the RHS, state_hash_work thread travels the old bydst and tranfers
every xfrm_state (include x) to the new bydst hlist and new byspi hlist;
3. user thread on the LHS gets the lock and adds x to the new byspi
hlist again.

thanks




On Wed, 2020-10-21 at 18:12 +1100, Herbert Xu wrote:
> On Tue, Oct 20, 2020 at 04:18:00PM +0800, Zhuoliang Zhang wrote:
> > From: zhuoliang zhang <zhuoliang.zhang at mediatek.com>
> > 
> > we found that the following race condition exists in
> > xfrm_alloc_userspi flow:
> > 
> > user thread                                    state_hash_work thread
> > ----                                           ----
> > xfrm_alloc_userspi()
> >  __find_acq_core()
> >    /*alloc new xfrm_state:x*/
> >    xfrm_state_alloc()
> >    /*schedule state_hash_work thread*/
> >    xfrm_hash_grow_check()   	               xfrm_hash_resize()
> >  xfrm_alloc_spi                                  /*hold lock*/
> >       x->id.spi = htonl(spi)                     spin_lock_bh(&net->xfrm.xfrm_state_lock)
> >       /*waiting lock release*/                     xfrm_hash_transfer()
> >       spin_lock_bh(&net->xfrm.xfrm_state_lock)      /*add x into hlist:net->xfrm.state_byspi*/
> > 	                                                hlist_add_head_rcu(&x->byspi)
> >                                                  spin_unlock_bh(&net->xfrm.xfrm_state_lock)
> > 
> >     /*add x into hlist:net->xfrm.state_byspi 2 times*/
> >     hlist_add_head_rcu(&x->byspi)
> > 
> > So the same xfrm_stame (x) is added into the same list_hash
> > (net->xfrm.state_byspi)2 times that makes the list_hash become
> > a inifite loop.
> 
> Your explanation makes no sense.  Prior to obtaining the spin lock
> on the LHS, the state x hasn't been added to thte table yet.  So
> how can the RHS be transferring it?
> 
> Cheers,