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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     linux-kernel@vger.kernel.org, x86@kernel.org
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, James Morse <James.Morse@arm.com>
Subject: [PATCH 2/2] x86/intel_rdt: Plug task_work vs task_struct {rmid,closid} update race
Date:   Wed, 18 Nov 2020 18:00:30 +0000
Message-Id: <20201118180030.22764-3-valentin.schneider@arm.com>
In-Reply-To: <20201118180030.22764-1-valentin.schneider@arm.com>
References: <20201118180030.22764-1-valentin.schneider@arm.com>
MIME-Version: 1.0
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Upon moving a task to a new control / monitor group, said task's {closid,
rmid} fields are updated *after* triggering the move_myself() task_work
callback. This can cause said callback to miss the update, e.g. if the
triggering thread got preempted before fiddling with task_struct, or if the
targeted task was already on its way to return to userspace.

Update the task_struct's {closid, rmid} tuple *before* invoking
task_work_add(). As they can happen concurrently, wrap {closid, rmid}
accesses with READ_ONCE() and WRITE_ONCE(). Highlight the required ordering
with a pair of comments.

Fixes: e02737d5b826 ("x86/intel_rdt: Add tasks files")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 arch/x86/include/asm/resctrl.h         | 11 ++++--
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 54 +++++++++++++++-----------
 2 files changed, 39 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
index 07603064df8f..d60ed0668a59 100644
--- a/arch/x86/include/asm/resctrl.h
+++ b/arch/x86/include/asm/resctrl.h
@@ -56,19 +56,22 @@ static void __resctrl_sched_in(void)
 	struct resctrl_pqr_state *state = this_cpu_ptr(&pqr_state);
 	u32 closid = state->default_closid;
 	u32 rmid = state->default_rmid;
+	u32 tmp;
 
 	/*
 	 * If this task has a closid/rmid assigned, use it.
 	 * Else use the closid/rmid assigned to this cpu.
 	 */
 	if (static_branch_likely(&rdt_alloc_enable_key)) {
-		if (current->closid)
-			closid = current->closid;
+		tmp = READ_ONCE(current->closid);
+		if (tmp)
+			closid = tmp;
 	}
 
 	if (static_branch_likely(&rdt_mon_enable_key)) {
-		if (current->rmid)
-			rmid = current->rmid;
+		tmp = READ_ONCE(current->rmid);
+		if (tmp)
+			rmid = tmp;
 	}
 
 	if (closid != state->cur_closid || rmid != state->cur_rmid) {
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index b6b5b95df833..135a51529f70 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -524,11 +524,13 @@ static void move_myself(struct callback_head *head)
 	 * If resource group was deleted before this task work callback
 	 * was invoked, then assign the task to root group and free the
 	 * resource group.
+	 *
+	 * See pairing atomic_inc() in __rdtgroup_move_task()
 	 */
 	if (atomic_dec_and_test(&rdtgrp->waitcount) &&
 	    (rdtgrp->flags & RDT_DELETED)) {
-		current->closid = 0;
-		current->rmid = 0;
+		WRITE_ONCE(current->closid, 0);
+		WRITE_ONCE(current->rmid, 0);
 		kfree(rdtgrp);
 	}
 
@@ -553,14 +555,32 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
 	callback = kzalloc(sizeof(*callback), GFP_KERNEL);
 	if (!callback)
 		return -ENOMEM;
-	callback->work.func = move_myself;
+
+	init_task_work(&callback->work, move_myself);
 	callback->rdtgrp = rdtgrp;
 
+	/*
+	 * For ctrl_mon groups move both closid and rmid.
+	 * For monitor groups, can move the tasks only from
+	 * their parent CTRL group.
+	 */
+	if (rdtgrp->type == RDTCTRL_GROUP)
+		WRITE_ONCE(tsk->closid, rdtgrp->closid);
+	WRITE_ONCE(tsk->rmid, rdtgrp->mon.rmid);
+
 	/*
 	 * Take a refcount, so rdtgrp cannot be freed before the
 	 * callback has been invoked.
+	 *
+	 * Also ensures above {closid, rmid} writes are observed by
+	 * move_myself(), as it can run immediately after task_work_add().
+	 * Otherwise old values may be loaded, and the move will only actually
+	 * happen at the next context switch.
+	 *
+	 * Pairs with atomic_dec() in move_myself().
 	 */
 	atomic_inc(&rdtgrp->waitcount);
+
 	ret = task_work_add(tsk, &callback->work, TWA_RESUME);
 	if (ret) {
 		/*
@@ -571,18 +591,6 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
 		atomic_dec(&rdtgrp->waitcount);
 		kfree(callback);
 		rdt_last_cmd_puts("Task exited\n");
-	} else {
-		/*
-		 * For ctrl_mon groups move both closid and rmid.
-		 * For monitor groups, can move the tasks only from
-		 * their parent CTRL group.
-		 */
-		if (rdtgrp->type == RDTCTRL_GROUP) {
-			tsk->closid = rdtgrp->closid;
-			tsk->rmid = rdtgrp->mon.rmid;
-		} else if (rdtgrp->type == RDTMON_GROUP) {
-			tsk->rmid = rdtgrp->mon.rmid;
-		}
 	}
 	return ret;
 }
@@ -590,13 +598,15 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
 static bool is_closid_match(struct task_struct *t, struct rdtgroup *r)
 {
 	return (rdt_alloc_capable &&
-	       (r->type == RDTCTRL_GROUP) && (t->closid == r->closid));
+		(r->type == RDTCTRL_GROUP) &&
+		(READ_ONCE(t->closid) == r->closid));
 }
 
 static bool is_rmid_match(struct task_struct *t, struct rdtgroup *r)
 {
 	return (rdt_mon_capable &&
-	       (r->type == RDTMON_GROUP) && (t->rmid == r->mon.rmid));
+		(r->type == RDTMON_GROUP) &&
+		(READ_ONCE(t->rmid) == r->mon.rmid));
 }
 
 /**
@@ -672,7 +682,7 @@ static int rdtgroup_move_task(pid_t pid, struct rdtgroup *rdtgrp,
 		goto out;
 
 	if (rdtgrp->type == RDTMON_GROUP &&
-	    rdtgrp->mon.parent->closid != tsk->closid) {
+	    rdtgrp->mon.parent->closid != READ_ONCE(tsk->closid)) {
 		rdt_last_cmd_puts("Can't move task to different control group\n");
 		ret = -EINVAL;
 		goto out;
@@ -802,7 +812,7 @@ int proc_resctrl_show(struct seq_file *s, struct pid_namespace *ns,
 		    rdtg->mode != RDT_MODE_EXCLUSIVE)
 			continue;
 
-		if (rdtg->closid != tsk->closid)
+		if (rdtg->closid != READ_ONCE(tsk->closid))
 			continue;
 
 		seq_printf(s, "res:%s%s\n", (rdtg == &rdtgroup_default) ? "/" : "",
@@ -810,7 +820,7 @@ int proc_resctrl_show(struct seq_file *s, struct pid_namespace *ns,
 		seq_puts(s, "mon:");
 		list_for_each_entry(crg, &rdtg->mon.crdtgrp_list,
 				    mon.crdtgrp_list) {
-			if (tsk->rmid != crg->mon.rmid)
+			if (READ_ONCE(tsk->rmid) != crg->mon.rmid)
 				continue;
 			seq_printf(s, "%s", crg->kn->name);
 			break;
@@ -2328,8 +2338,8 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
 	for_each_process_thread(p, t) {
 		if (!from || is_closid_match(t, from) ||
 		    is_rmid_match(t, from)) {
-			t->closid = to->closid;
-			t->rmid = to->mon.rmid;
+			WRITE_ONCE(t->closid, to->closid);
+			WRITE_ONCE(t->rmid, to->mon.rmid);
 
 #ifdef CONFIG_SMP
 			/*
-- 
2.27.0