[PATCH v2 04/21] sunrpc: defer rq_argp and rq_resp free until after RCU grace period

[Jeff Layton <jlayton@kernel.org>]

svc_rqst_free() frees rqstp->rq_argp and rqstp->rq_resp synchronously
via kfree(), but defers the rqstp struct free via kfree_rcu().  After
svc_exit_thread() calls list_del_rcu() and svc_rqst_free(), there is
a window where RCU readers that started before list_del_rcu() can still
traverse the thread list and find the rqstp.  These readers (e.g.
nfsd_nl_rpc_status_get_dumpit()) dereference rqstp->rq_argp, which has
already been freed — a use-after-free.

Fix this by moving the kfree of rq_argp and rq_resp into an explicit
call_rcu() callback alongside the struct free.  Resources not accessed
by RCU readers (bvec, buffer pages, scratch folio, auth_data) remain
synchronously freed.

Fixes: 812443865c5f ("sunrpc: add a rcu_head to svc_rqst and use kfree_rcu to free it")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 net/sunrpc/svc.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 86d39610cf0a..dd80a2eaaa74 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -716,6 +716,15 @@ svc_release_buffer(struct svc_rqst *rqstp)
 	}
 }
 
+static void svc_rqst_free_rcu(struct rcu_head *head)
+{
+	struct svc_rqst *rqstp = container_of(head, struct svc_rqst, rq_rcu_head);
+
+	kfree(rqstp->rq_resp);
+	kfree(rqstp->rq_argp);
+	kfree(rqstp);
+}
+
 static void
 svc_rqst_free(struct svc_rqst *rqstp)
 {
@@ -724,10 +733,8 @@ svc_rqst_free(struct svc_rqst *rqstp)
 	svc_release_buffer(rqstp);
 	if (rqstp->rq_scratch_folio)
 		folio_put(rqstp->rq_scratch_folio);
-	kfree(rqstp->rq_resp);
-	kfree(rqstp->rq_argp);
 	kfree(rqstp->rq_auth_data);
-	kfree_rcu(rqstp, rq_rcu_head);
+	call_rcu(&rqstp->rq_rcu_head, svc_rqst_free_rcu);
 }
 
 static struct svc_rqst *

-- 
2.54.0