[PATCH net v2] xfrm: retry inexact policy lookup after node reinsertion

[Chengfeng Ye <nicoyip.dev@gmail.com>]

An inexact policy lookup first records pointers to candidate hlist heads
and then traverses the lists.  A concurrent policy insertion can merge
inexact tree nodes between those operations:

  lookup                         policy insertion
  ------                         ----------------
  find inexact candidates
    save obsolete hlist head
                                 write_seqcount_begin(&bin->count)
                                 merge inexact tree nodes
                                   hlist_del_rcu(&policy->bydst)
                                   reinsert policy->bydst in survivor
                                 write_seqcount_end(&bin->count)
  evaluate saved candidate list
    miss the moved policy

The merge immediately reinserts the same hlist node into the surviving
tree node.  RCU keeps the policy alive, but it does not provide a
consistent view while its list node is moved.  A lookup that selected the
obsolete list can observe it empty.  A lookup already traversing a moved
policy can instead follow the next pointer rewritten by the reinsertion.
Either case can return an incorrect IPsec policy result.

The per-bin sequence counter already brackets calls to
xfrm_policy_inexact_insert_node(), including node merges.  The read side,
however, currently validates the counter only while searching an
individual rb-tree.  A successful search returns without validation, and
the later candidate-list traversal is outside that read-side section.

Snapshot the per-bin sequence before discovering candidate heads and
validate it after evaluating all candidate lists.   Retry the lookup if
the inexact policy tree changes while it is being searched.

Fixes: 9cf545ebd591 ("xfrm: policy: store inexact policies in a tree ordered by destination address")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
Changes in v2:
- (changelog) Drop the unrelated xfrm_policy_count[] KCSAN report.

v1: https://lore.kernel.org/netdev/20260824152057.216329-1-nicoyip.dev@gmail.com/

 net/xfrm/xfrm_policy.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 932a313b9460..5d4e863863df 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2157,6 +2157,7 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type,
 	struct xfrm_pol_inexact_bin *bin;
 	struct xfrm_policy *pol, *ret;
 	struct hlist_head *chain;
+	unsigned int inexact_sequence;
 	unsigned int sequence;
 	int err;
 
@@ -2191,12 +2192,18 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type,
 		goto skip_inexact;
 
 	bin = xfrm_policy_inexact_lookup_rcu(net, type, family, dir, if_id);
-	if (!bin || !xfrm_policy_find_inexact_candidates(&cand, bin, saddr,
-							 daddr))
+	if (!bin)
+		goto skip_inexact;
+
+	inexact_sequence = read_seqcount_begin(&bin->count);
+	if (!xfrm_policy_find_inexact_candidates(&cand, bin, saddr, daddr))
 		goto skip_inexact;
 
 	pol = xfrm_policy_eval_candidates(&cand, ret, fl, type,
 					  family, if_id);
+	if (read_seqcount_retry(&bin->count, inexact_sequence))
+		goto retry;
+
 	if (pol) {
 		ret = pol;
 		if (IS_ERR(pol))
-- 
2.43.0