[PATCH 3/6] xfrm: policy: fix use-after-free on inexact bin in xfrm_policy_bysel_ctx()

[Steffen Klassert <steffen.klassert@secunet.com>]

From: Sanghyun Park <sanghyun.park.cnu@gmail.com>

Fix the race by pruning the bin while still holding xfrm_policy_lock,
before dropping it. Use __xfrm_policy_inexact_prune_bin() directly since
the lock is already held. The wrapper xfrm_policy_inexact_prune_bin()
becomes unused and is removed.

Race:

  CPU0 (XFRM_MSG_DELPOLICY)           CPU1 (XFRM_MSG_NEWSPDINFO)
  ==========================          ==========================
  xfrm_policy_bysel_ctx():
    spin_lock_bh(xfrm_policy_lock)
    bin = xfrm_policy_inexact_lookup()
    __xfrm_policy_unlink(pol)
    spin_unlock_bh(xfrm_policy_lock)
    xfrm_policy_kill(ret)
    // wide window, lock not held
                                       xfrm_hash_rebuild():
                                         spin_lock_bh(xfrm_policy_lock)
                                         __xfrm_policy_inexact_flush():
                                           kfree_rcu(bin)  // bin freed
                                         spin_unlock_bh(xfrm_policy_lock)
    xfrm_policy_inexact_prune_bin(bin)
    // UAF: bin is freed

Fixes: 6be3b0db6db8 ("xfrm: policy: add inexact policy search tree infrastructure")
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
---
 net/xfrm/xfrm_policy.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index dd09d2063da2..959544425692 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -1156,15 +1156,6 @@ static void __xfrm_policy_inexact_prune_bin(struct xfrm_pol_inexact_bin *b, bool
 	}
 }
 
-static void xfrm_policy_inexact_prune_bin(struct xfrm_pol_inexact_bin *b)
-{
-	struct net *net = read_pnet(&b->k.net);
-
-	spin_lock_bh(&net->xfrm.xfrm_policy_lock);
-	__xfrm_policy_inexact_prune_bin(b, false);
-	spin_unlock_bh(&net->xfrm.xfrm_policy_lock);
-}
-
 static void __xfrm_policy_inexact_flush(struct net *net)
 {
 	struct xfrm_pol_inexact_bin *bin, *t;
@@ -1707,12 +1698,12 @@ xfrm_policy_bysel_ctx(struct net *net, const struct xfrm_mark *mark, u32 if_id,
 		}
 		ret = pol;
 	}
+	if (bin && delete)
+		__xfrm_policy_inexact_prune_bin(bin, false);
 	spin_unlock_bh(&net->xfrm.xfrm_policy_lock);
 
 	if (ret && delete)
 		xfrm_policy_kill(ret);
-	if (bin && delete)
-		xfrm_policy_inexact_prune_bin(bin);
 	return ret;
 }
 EXPORT_SYMBOL(xfrm_policy_bysel_ctx);
-- 
2.43.0