[PATCH 2/3] nfsd: fix UAF in cpntf statelist drain

From: Chris Mason <[email protected]>

nfs4_free_cpntf_statelist() drains a parent stid's sc_cp_list by
repeatedly taking the first entry and calling
_free_cpntf_state_locked(), which only unlinks and frees the entry
when refcount_dec_and_test() drops cs_count to zero.  When a
concurrent holder has bumped cs_count via manage_cpntf_state(),
the helper returns early and leaves the entry on sc_cp_list, so
the drain re-decrements the same entry on the next iteration and
burns the reference that belonged to that concurrent holder.

    CPU 0                              CPU 1
    -----                              -----
    find_cpntf_state()
      manage_cpntf_state(clp=NULL)
        spin_lock(&nn->s2s_cp_lock)
        refcount_inc(cs_count) 1->2
        spin_unlock(&nn->s2s_cp_lock)
      /* caller now owns a ref */
                                       nfs4_free_cpntf_statelist()
                                         spin_lock(&s2s_cp_lock)
                                         iter 1: dec_and_test 2->1
                                                 (no unlink/free)
                                         iter 2: dec_and_test 1->0
                                                 list_del + idr_remove
                                                 kfree(cps)
                                         spin_unlock(&s2s_cp_lock)
    nfs4_put_cpntf_state(cps)
      spin_lock(&s2s_cp_lock)
      _free_cpntf_state_locked(cps)    /* cps is freed slab */

The late put writes into the freed slab object's refcount_t, a
KASAN-detectable use-after-free.

Fix by rewriting the drain to unconditionally revoke each entry
before dropping the parent's reference.  Walk sc_cp_list with
list_for_each_entry_safe(), list_del_init() the entry, remove it
from nn->s2s_cp_stateids, and only then drop the parent-owned
reference via a new put_cpntf_state_unlinked_locked() helper
that does refcount_dec_and_test() + kfree().  The drain now
terminates in one pass per entry regardless of cs_count.

Concurrent holders keep their own reference; their eventual
nfs4_put_cpntf_state() reaches _free_cpntf_state_locked() on an
entry the drain has already unlinked from sc_cp_list and from
nn->s2s_cp_stateids.  _free_cpntf_state_locked() gates its
list_del_init() and idr_remove() on !list_empty(&cps->cp_list);
the drain's list_del_init() leaves cp_list self-linked, so the
late put skips both calls and only the final refcount drop runs.

Gating idr_remove() on the same check is required because
idr_alloc_cyclic() may already have recycled the so_id for an
unrelated cpntf entry by the time the late put runs; an
unconditional idr_remove() would silently unhash that live entry
and corrupt subsequent find_cpntf_state() lookups.

Fixes: 624322f1adc5 ("NFSD add COPY_NOTIFY operation")
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <[email protected]>
---
 fs/nfsd/nfs4state.c | 65 ++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 52 insertions(+), 13 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index b8946db3ebaa..374155e57f3f 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1025,17 +1025,58 @@ void nfs4_free_copy_state(struct nfsd4_copy *copy)
 	spin_unlock(&nn->s2s_cp_lock);
 }
 
+/*
+ * Drop the parent stid's reference on a cpntf entry that has already been
+ * removed from sc_cp_list and the s2s_cp_stateids IDR. If a concurrent holder
+ * still owns a reference (acquired viamanage_cpntf_state() before the unlink),
+ * that holder's nfs4_put_cpntf_state() will perform the final free.
+ *
+ * The nn->s2s_cp_lock must be held!
+ */
+static void put_cpntf_state_unlinked_locked(struct nfs4_cpntf_state *cps)
+{
+	WARN_ON_ONCE(cps->cp_stateid.cs_type != NFS4_COPYNOTIFY_STID);
+	WARN_ON_ONCE(!list_empty(&cps->cp_list));
+
+	if (refcount_dec_and_test(&cps->cp_stateid.cs_count))
+		kfree(cps);
+}
+
+/*
+ * Unhash the stateid from the s2s stateid hash, and detach it from the sc_cp_list.
+ * Note that this is gated on a list_empty() check, to avoid problems with IDR
+ * hashval reuse.
+ */
+static void nfsd4_unhash_cpntf_state(struct nfsd_net *nn, struct nfs4_cpntf_state *cps)
+{
+	lockdep_assert_held(&nn->s2s_cp_lock);
+
+	if (!list_empty(&cps->cp_list)) {
+		list_del_init(&cps->cp_list);
+		idr_remove(&nn->s2s_cp_stateids, cps->cp_stateid.cs_stid.si_opaque.so_id);
+	}
+}
+
 static void nfs4_free_cpntf_statelist(struct net *net, struct nfs4_stid *stid)
 {
-	struct nfs4_cpntf_state *cps;
+	struct nfs4_cpntf_state *cps, *tmp;
 	struct nfsd_net *nn;
 
 	nn = net_generic(net, nfsd_net_id);
 	spin_lock(&nn->s2s_cp_lock);
-	while (!list_empty(&stid->sc_cp_list)) {
-		cps = list_first_entry(&stid->sc_cp_list,
-				       struct nfs4_cpntf_state, cp_list);
-		_free_cpntf_state_locked(nn, cps);
+	/*
+	 * Unlink every entry from sc_cp_list and the IDR before dropping
+	 * the parent's reference.  This makes the drain terminate in one
+	 * pass per entry regardless of cs_count: a concurrent holder that
+	 * obtained the entry via manage_cpntf_state() retains its own
+	 * reference, and its eventual nfs4_put_cpntf_state() will see the
+	 * entry already unlinked (list_del_init() in
+	 * _free_cpntf_state_locked makes that a no-op) and drive the final
+	 * kfree itself.
+	 */
+	list_for_each_entry_safe(cps, tmp, &stid->sc_cp_list, cp_list) {
+		nfsd4_unhash_cpntf_state(nn, cps);
+		put_cpntf_state_unlinked_locked(cps);
 	}
 	spin_unlock(&nn->s2s_cp_lock);
 }
@@ -7870,16 +7911,14 @@ nfs4_check_file(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfs4_stid *s,
 out:
 	return status;
 }
-static void
-_free_cpntf_state_locked(struct nfsd_net *nn, struct nfs4_cpntf_state *cps)
+
+static void _free_cpntf_state_locked(struct nfsd_net *nn, struct nfs4_cpntf_state *cps)
 {
 	WARN_ON_ONCE(cps->cp_stateid.cs_type != NFS4_COPYNOTIFY_STID);
-	if (!refcount_dec_and_test(&cps->cp_stateid.cs_count))
-		return;
-	list_del_init(&cps->cp_list);
-	idr_remove(&nn->s2s_cp_stateids,
-		   cps->cp_stateid.cs_stid.si_opaque.so_id);
-	kfree(cps);
+	if (refcount_dec_and_test(&cps->cp_stateid.cs_count)) {
+		nfsd4_unhash_cpntf_state(nn, cps);
+		kfree(cps);
+	}
 }
 /*
  * A READ from an inter server to server COPY will have a

-- 
2.54.0