linux-kernel - [tip:smp/hotplug] smp/hotplug: Handle removal correctly in cpuhp_store

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Date:   Thu, 20 Jul 2017 07:49:09 -0700
From:   tip-bot for Ethan Barnes <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     srivatsa@....edu, bigeasy@...utronix.d, mingo@...nel.or,
        mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        Ethan.Barnes@....com, hpa@...or.com, paulmck@...ux.vnet.ibm.com,
        ethan.barnes@...disk.com
Subject: [tip:smp/hotplug] smp/hotplug: Handle removal correctly in
 cpuhp_store_callbacks()

Commit-ID:  0c96b27305faf06c068b45e07d28336c80dac286
Gitweb:     http://git.kernel.org/tip/0c96b27305faf06c068b45e07d28336c80dac286
Author:     Ethan Barnes <Ethan.Barnes@....com>
AuthorDate: Wed, 19 Jul 2017 22:36:00 +0000
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 20 Jul 2017 16:40:24 +0200

smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()

If cpuhp_store_callbacks() is called for CPUHP_AP_ONLINE_DYN or
CPUHP_BP_PREPARE_DYN, which are the indicators for dynamically allocated
states, then cpuhp_store_callbacks() allocates a new dynamic state. The
first allocation in each range returns CPUHP_AP_ONLINE_DYN or
CPUHP_BP_PREPARE_DYN.

If cpuhp_remove_state() is invoked for one of these states, then there is
no protection against the allocation mechanism. So the removal, which
should clear the callbacks and the name, gets a new state assigned and
clears that one.

As a consequence the state which should be cleared stays initialized. A
consecutive CPU hotplug operation dereferences the state callbacks and
accesses either freed or reused memory, resulting in crashes.

Add a protection against this by checking the name argument for NULL. If
it's NULL it's a removal. If not, it's an allocation.

[ tglx: Added a comment and massaged changelog ]

Fixes: 5b7aa87e0482 ("cpu/hotplug: Implement setup/removal interface")
Signed-off-by: Ethan Barnes <ethan.barnes@...disk.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.or>
Cc: "Srivatsa S. Bhat" <srivatsa@....edu>
Cc: Sebastian Siewior <bigeasy@...utronix.d>
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: stable@...r.kernel.org
Link: http://lkml.kernel.org/r/DM2PR04MB398242FC7776D603D9F99C894A60@DM2PR04MB398.namprd04.prod.outlook.com
---
 kernel/cpu.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index eee0331..a88c29a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1252,7 +1252,17 @@ static int cpuhp_store_callbacks(enum cpuhp_state state, const char *name,
 	struct cpuhp_step *sp;
 	int ret = 0;
 
-	if (state == CPUHP_AP_ONLINE_DYN || state == CPUHP_BP_PREPARE_DYN) {
+	/*
+	 * If name is NULL, then the state gets removed.
+	 *
+	 * CPUHP_AP_ONLINE_DYN and CPUHP_BP_PREPARE_DYN are handed out on
+	 * the first allocation from these dynamic ranges, so the removal
+	 * would trigger a new allocation and clear the wrong (already
+	 * empty) state, leaving the callbacks of the to be cleared state
+	 * dangling, which causes wreckage on the next hotplug operation.
+	 */
+	if (name && (state == CPUHP_AP_ONLINE_DYN ||
+		     state == CPUHP_BP_PREPARE_DYN)) {
 		ret = cpuhp_reserve_state(state);
 		if (ret < 0)
 			return ret;