On Thu, Oct 22, 2020 at 06:01:27PM +0800, Zhuoliang Zhang wrote:

From: zhuoliang zhang <redacted>

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 <redacted>
---
 net/xfrm/xfrm_state.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
-- 
Email: Herbert Xu [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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