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

[Herbert Xu]

On Tue, Oct 20, 2020 at 04:18:00PM +0800, Zhuoliang Zhang wrote:
> From: zhuoliang zhang <zhuoliang.zhang@xxxxxxxxxxxx>
> 
> 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,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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