The RESCTRL_GLOBAL_ENABLE_PER_CPU kernel mode can be backed by only one
resource group at a time. The active kernel-mode association depends on
the group's mode and, for a monitor group, its parent control group.

Prevent mode changes for a resource group that backs the active kernel
mode. Also prevent mode changes to the parent control group of a backing
monitor group, and prevent a backing monitor group from being moved to a
different parent. These restrictions ensure that the active kernel-mode
association remains valid and consistent.

Signed-off-by: Babu Moger <babu.moger@amd.com>
---
v6: New patch to fix sashiko feedback.
    https://sashiko.dev/#/patchset/cover.1787772750.git.babu.moger%40amd.com
---
 fs/resctrl/internal.h |  2 ++
 fs/resctrl/rdtgroup.c | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
index 6b8d14c4bd83..84238e9e8e15 100644
--- a/fs/resctrl/internal.h
+++ b/fs/resctrl/internal.h
@@ -462,6 +462,8 @@ ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of, char *buf,
 void resctrl_bmec_files_show(struct rdt_resource *r, struct kernfs_node *l3_mon_kn,
 			     bool show);
 
+bool rdtgroup_has_kmode_global(struct rdtgroup *rdtgrp);
+
 int resctrl_num_mbm_cntrs_show(struct kernfs_open_file *of, struct seq_file *s, void *v);
 
 int resctrl_available_mbm_cntrs_show(struct kernfs_open_file *of, struct seq_file *s,
diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 98451b417fe3..dcf9f26540d6 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -1738,6 +1738,27 @@ static bool rdtgroup_mode_test_exclusive(struct rdtgroup *rdtgrp)
 	return true;
 }
 
+/* True when @rdtgrp or one of its monitor groups backs the kernel mode. */
+bool rdtgroup_has_kmode_global(struct rdtgroup *rdtgrp)
+{
+	struct rdtgroup *child;
+
+	if (!resctrl_kmode.rdtgrp)
+		return false;
+
+	if (resctrl_kmode.rdtgrp == rdtgrp)
+		return true;
+
+	if (rdtgrp->type != RDTCTRL_GROUP)
+		return false;
+
+	list_for_each_entry(child, &rdtgrp->mon.crdtgrp_list, mon.crdtgrp_list)
+		if (child == resctrl_kmode.rdtgrp)
+			return true;
+
+	return false;
+}
+
 /*
  * rdtgroup_mode_write - Modify the resource group's mode
  */
@@ -1778,6 +1799,12 @@ static ssize_t rdtgroup_mode_write(struct kernfs_open_file *of,
 		goto out;
 	}
 
+	if (rdtgroup_has_kmode_global(rdtgrp)) {
+		rdt_last_cmd_puts("Cannot change mode for kernel-mode assigned group\n");
+		ret = -EINVAL;
+		goto out;
+	}
+
 	if (!strcmp(buf, "shareable")) {
 		if (rdtgrp->mode == RDT_MODE_PSEUDO_LOCKSETUP) {
 			ret = rdtgroup_locksetup_exit(rdtgrp);
@@ -4697,6 +4724,14 @@ static int rdtgroup_rename(struct kernfs_node *kn,
 		goto out;
 	}
 
+	/* Reject moving to different parent if group backs kernel-mode */
+	if (rdtgrp == resctrl_kmode.rdtgrp &&
+	    rdtgrp->mon.parent != new_prdtgrp) {
+		rdt_last_cmd_puts("Cannot move a group that backs kernel mode\n");
+		ret = -EPERM;
+		goto out;
+	}
+
 	/*
 	 * Allocate the cpumask for use in mongrp_reparent() to avoid the
 	 * possibility of failing to allocate it after kernfs_rename() has
-- 
2.43.0