Commit a779d913 authored Oct 23, 2020 by zhuoliang zhang Committed by Steffen Klassert Oct 23, 2020

net: xfrm: fix a race condition during allocing spi



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: f034b5d4 ("[XFRM]: Dynamic xfrm_state hash table sizing.")
Signed-off-by: zhuoliang zhang <zhuoliang.zhang@mediatek.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

parent 7fe94612

net/xfrm/xfrm_state.c

+5 −3

Original line number	Diff line number	Diff line
		@@ -2004,6 +2004,7 @@ int xfrm_alloc_spi(struct xfrm_state *x, u32 low, u32 high)
		int err = -ENOENT;
		__be32 minspi = htonl(low);
		__be32 maxspi = htonl(high);
		__be32 newspi = 0;
		u32 mark = x->mark.v & x->mark.m;

		spin_lock_bh(&x->lock);
		@@ -2022,21 +2023,22 @@ int xfrm_alloc_spi(struct xfrm_state *x, u32 low, u32 high)
		xfrm_state_put(x0);
		goto unlock;
		}
		x->id.spi = minspi;
		newspi = minspi;
		} else {
		u32 spi = 0;
		for (h = 0; h < high-low+1; h++) {
		spi = low + prandom_u32()%(high-low+1);
		x0 = xfrm_state_lookup(net, mark, &x->id.daddr, htonl(spi), x->id.proto, x->props.family);
		if (x0 == NULL) {
		x->id.spi = htonl(spi);
		newspi = htonl(spi);
		break;
		}
		xfrm_state_put(x0);
		}
		}
		if (x->id.spi) {
		if (newspi) {
		spin_lock_bh(&net->xfrm.xfrm_state_lock);
		x->id.spi = newspi;
		h = xfrm_spi_hash(net, &x->id.daddr, x->id.spi, x->id.proto, x->props.family);
		hlist_add_head_rcu(&x->byspi, net->xfrm.state_byspi + h);
		spin_unlock_bh(&net->xfrm.xfrm_state_lock);

Admin message