[PATCH v2] NFS: defer access-cache entry and group_info free past RCU

[Cheng Yongkang <teel4res@gmail.com>]

nfs_access_free_entry() drops the group_info reference synchronously
while the nfs_access_entry itself is only freed after an RCU grace
period.  The lockless lookup path (nfs_access_get_cached_rcu() ->
access_cmp()) can still be walking group_info->ngroups/gid[] after the
entry was replaced and unlinked, so put_group_info() can reach
groups_free() -> kvfree() under a reader and free the array it is
reading: a slab-use-after-free read.

put_group_info() may end in kvfree(), so it cannot run from a plain RCU
callback.  Defer both the group_info reference drop and the entry free
to process context after a grace period with queue_rcu_work() on
nfsiod_workqueue.  The entry accounting (nfs_access_nr_entries) and all
call-site locking are unchanged; the work only drops the reference and
frees the entry.

On a 7.3 KASAN kernel, the following workload triggers the report: a
userspace NFSv3 server plus eight readers doing faccessat() with 2048
supplementary gids, and serialized credential replacements that detach
the previous killer's group_info reference.

 BUG: KASAN: use-after-free in nfs_access_get_cached+0xb8f/0xf80
 Read of size 4 at addr ... by task poc/9552
 Call Trace:
  nfs_access_get_cached
  nfs_do_access
  nfs_permission
  inode_permission
  do_faccessat
 Page last allocated by task ...:
  __kvmalloc_node_noprof
  __se_sys_setgroups
 Page last freed by task ...:
  nfs_access_free_entry
  nfs_access_add_cache
  nfs_do_access

The trigger needs CAP_SETGID in the guest for the setgroups-based
credential churn; the faulty read path is the ordinary access(2) /
faccessat(2) path used by any NFS user.

Link: https://lore.kernel.org/all/20261025180329.349613-1-teel4res@gmail.com/
Fixes: 6238aec83f3f ("NFS: don't store 'struct cred *' in struct nfs_access_entry")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Cheng Yongkang <teel4res@gmail.com>
---
v2: add Link, Fixes and stable tags, and remove local validation details from the
commit message.

 fs/nfs/dir.c           | 21 +++++++++++++++++----
 include/linux/nfs_fs.h |  3 ++-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -2898,10 +2898,25 @@
 module_param(nfs_access_max_cachesize, ulong, 0644);
 MODULE_PARM_DESC(nfs_access_max_cachesize, "NFS access maximum total cache length");

-static void nfs_access_free_entry(struct nfs_access_entry *entry)
+static void nfs_access_free_entry_work(struct work_struct *work)
 {
+	struct nfs_access_entry *entry =
+		container_of(to_rcu_work(work), struct nfs_access_entry, rcu_work);
+
 	put_group_info(entry->group_info);
-	kfree_rcu(entry, rcu_head);
+	kfree(entry);
+}
+
+static void nfs_access_free_entry(struct nfs_access_entry *entry)
+{
+	/*
+	 * The entry can still be reached by RCU readers that dereference
+	 * its group_info in access_cmp().  Drop the group_info reference
+	 * only after a grace period, in process context because
+	 * put_group_info() may end in kvfree().
+	 */
+	INIT_RCU_WORK(&entry->rcu_work, nfs_access_free_entry_work);
+	queue_rcu_work(nfsiod_workqueue, &entry->rcu_work);
 	smp_mb__before_atomic();
 	atomic_long_dec(&nfs_access_nr_entries);
 	smp_mb__after_atomic();
diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -26,6 +26,7 @@
 #include <linux/refcount.h>
 #include <linux/rwsem.h>
 #include <linux/wait.h>
+#include <linux/workqueue.h>

 #include <linux/sunrpc/debug.h>
 #include <linux/sunrpc/auth.h>
@@ -65,7 +66,7 @@
 	struct group_info	*group_info;
 	u64			timestamp;
 	__u32			mask;
-	struct rcu_head		rcu_head;
+	struct rcu_work		rcu_work;
 };

 struct nfs_lock_context {