[PATCH 4.4 06/64] net: xfrm: fix a race condition during allocing spi

Greg Kroah-Hartman
2020-11-17T13:07:44Z

From: zhuoliang zhang <zhuoliang.zhang@mediatek.com>

[ Upstream commit a779d91314ca7208b7feb3ad817b62904397c56d ]

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@mediatek.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/xfrm/xfrm_state.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 5bb5950d6276b..ed05cd7a4ef2c 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -1554,6 +1554,7 @@ 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);
@@ -1572,21 +1573,22 @@ 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(&x->byspi, net->xfrm.state_byspi+h);
spin_unlock_bh(&net->xfrm.xfrm_state_lock);
--
2.27.0