4.9-stable review patch. If anyone has any objections, please let me know.
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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 00 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:0000000000000000 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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/xen/cpu_hotplug.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-)
--- a/drivers/xen/cpu_hotplug.c +++ b/drivers/xen/cpu_hotplug.c @@ -18,15 +18,16 @@ static void enable_hotplug_cpu(int cpu)
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(); }
static int vcpu_online(unsigned int cpu)