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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Olaf Hering <olaf@aepfle.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL 4.4 16/17] xen: avoid crash in disable_hotplug_cpu
Thread-Topic: [PATCH AUTOSEL 4.4 16/17] xen: avoid crash in
 disable_hotplug_cpu
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Date:   Mon, 1 Oct 2018 00:41:34 +0000
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From: Olaf Hering <olaf@aepfle.de>

[ Upstream commit 3366cdb6d350d95466ee430ac50f3c8415ca8f46 ]

The command 'xl vcpu-set 0 0', issued in dom0, will crash dom0:

BUG: unable to handle kernel NULL pointer dereference at 00000000000002d8
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 7 PID: 65 Comm: xenwatch Not tainted 4.19.0-rc2-1.ga9462db-default #1 =
openSUSE Tumbleweed (unreleased)
Hardware name: Intel Corporation S5520UR/S5520UR, BIOS S5500.86B.01.00.0050=
.050620101605 05/06/2010
RIP: e030:device_offline+0x9/0xb0
Code: 77 24 00 e9 ce fe ff ff 48 8b 13 e9 68 ff ff ff 48 8b 13 e9 29 ff ff =
ff 48 8b 13 e9 ea fe ff ff 90 66 66 66 66 90 41 54 55 53 <f6> 87 d8 02 00 0=
0 01 0f 85 88 00 00 00 48 c7 c2 20 09 60 81 31 f6
RSP: e02b:ffffc90040f27e80 EFLAGS: 00010203
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff8801f3800000 RSI: ffffc90040f27e70 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff820e47b3 R09: 0000000000000000
R10: 0000000000007ff0 R11: 0000000000000000 R12: ffffffff822e6d30
R13: dead000000000200 R14: dead000000000100 R15: ffffffff8158b4e0
FS:  00007ffa595158c0(0000) GS:ffff8801f39c0000(0000) knlGS:000000000000000=
0
CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000002d8 CR3: 00000001d9602000 CR4: 0000000000002660
Call Trace:
 handle_vcpu_hotplug_event+0xb5/0xc0
 xenwatch_thread+0x80/0x140
 ? wait_woken+0x80/0x80
 kthread+0x112/0x130
 ? kthread_create_worker_on_cpu+0x40/0x40
 ret_from_fork+0x3a/0x50

This happens because handle_vcpu_hotplug_event is called twice. In the
first iteration cpu_present is still true, in the second iteration
cpu_present is false which causes get_cpu_device to return NULL.
In case of cpu#0, cpu_online is apparently always true.

Fix this crash by checking if the cpu can be hotplugged, which is false
for a cpu that was just removed.

Also check if the cpu was actually offlined by device_remove, otherwise
leave the cpu_present state as it is.

Rearrange to code to do all work with device_hotplug_lock held.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 drivers/xen/cpu_hotplug.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/xen/cpu_hotplug.c b/drivers/xen/cpu_hotplug.c
index 5676aefdf2bc..f4e59c445964 100644
--- a/drivers/xen/cpu_hotplug.c
+++ b/drivers/xen/cpu_hotplug.c
@@ -18,15 +18,16 @@ static void enable_hotplug_cpu(int cpu)
=20
 static void disable_hotplug_cpu(int cpu)
 {
-	if (cpu_online(cpu)) {
-		lock_device_hotplug();
+	if (!cpu_is_hotpluggable(cpu))
+		return;
+	lock_device_hotplug();
+	if (cpu_online(cpu))
 		device_offline(get_cpu_device(cpu));
-		unlock_device_hotplug();
-	}
-	if (cpu_present(cpu))
+	if (!cpu_online(cpu) && cpu_present(cpu)) {
 		xen_arch_unregister_cpu(cpu);
-
-	set_cpu_present(cpu, false);
+		set_cpu_present(cpu, false);
+	}
+	unlock_device_hotplug();
 }
=20
 static int vcpu_online(unsigned int cpu)
--=20
2.17.1