[RFC PATCH] fs/resctrl: Fix use-after-free during unmount

[Tony Luck <tony.luck@intel.com>]

Sashiko reported[1] this issue:

  During unmount or failure teardown, resctrl_fs_teardown() calls
  mon_put_kn_priv() (which frees all mon_data structures) followed
  by rdtgroup_destroy_root() (which destroys kernfs nodes). However, the
  RDT_DELETED flag is never set for rdtgroup_default.

  If a concurrent reader (e.g., rdtgroup_mondata_show()) invokes
  rdtgroup_kn_lock_live(), it drops kernfs active protection and blocks on
  rdtgroup_mutex. resctrl_fs_teardown() (holding the mutex) proceeds to free
  the private data and destroy the nodes without waiting for the reader.

  When the mutex is released, the reader wakes up, observes that RDT_DELETED is
  not set for the default group, and dereferences the already-freed of->kn->priv
  pointer.

Set RDT_DELETED for the default group (if there are any tasks waiting).

Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://sashiko.dev/#/patchset/20260508182143.14592-1-tony.luck%40intel.com?part=2 [1]
---

Yet another side-quest from Sashiko. RFC for some human eyes before I
add to my series and post a new version;

1) Is this real? It looks like it is to me.
2) Is my fix reasonable?
3) Better way to fix it?

 fs/resctrl/rdtgroup.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index eac7e4f8574d..668ebe0b0ec6 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -594,7 +594,8 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
 static void rdtgroup_remove(struct rdtgroup *rdtgrp)
 {
 	kernfs_put(rdtgrp->kn);
-	kfree(rdtgrp);
+	if (rdtgrp != &rdtgroup_default)
+		kfree(rdtgrp);
 }
 
 static void _update_task_closid_rmid(void *task)
@@ -2965,6 +2966,8 @@ static void resctrl_fs_teardown(void)
 	mon_put_kn_priv();
 	rdt_pseudo_lock_release();
 	rdtgroup_default.mode = RDT_MODE_SHAREABLE;
+	if (atomic_read(&rdtgroup_default.waitcount) != 0)
+		rdtgroup_default.flags = RDT_DELETED;
 	closid_exit();
 	schemata_list_destroy();
 	rdtgroup_destroy_root();
@@ -4291,6 +4294,7 @@ static int rdtgroup_setup_root(struct rdt_fs_context *ctx)
 
 	ctx->kfc.root = rdt_root;
 	rdtgroup_default.kn = kernfs_root_to_node(rdt_root);
+	rdtgroup_default.flags = 0;
 
 	return 0;
 }
-- 
2.54.0