Re: [PATCH v2] net: xfrm: fix a race condition during allocing spi

[Herbert Xu]

On Thu, Oct 22, 2020 at 06:01:27PM +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)
> 
> 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) into the new bydst hlist and new byspi hlist;
> 3. user thread on the LHS gets the lock and adds x into the new byspi hlist again.
> 
> So the same xfrm_state (x) is added into the same list_hash
> (net->xfrm.state_byspi) 2 times that makes the list_hash become
> an inifite loop.
> 
> To fix the race, x->id.spi = htonl(spi) in the xfrm_alloc_spi() is moved
> to the back of spin_lock_bh, sothat state_hash_work thread no longer add x
> which id.spi is zero into the hash_list.
> 
> Fixes: f034b5d4efdf ("[XFRM]: Dynamic xfrm_state hash table sizing.")
> Signed-off-by: zhuoliang zhang <zhuoliang.zhang@xxxxxxxxxxxx>
> ---
>  net/xfrm/xfrm_state.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)

Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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