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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Chris Co <chrco@microsoft.com>,
        "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>,
        Wei Liu <wei.liu@kernel.org>
Subject: [PATCH 5.4 148/158] Drivers: hv: vmbus: Allow cleanup of VMBUS_CONNECT_CPU if disconnected
Date:   Mon, 23 Nov 2020 13:22:56 +0100
Message-Id: <20201123121827.071521109@linuxfoundation.org>
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From: Chris Co <chrco@microsoft.com>

commit 92e4dc8b05663d6539b1b8375f3b1cf7b204cfe9 upstream.

When invoking kexec() on a Linux guest running on a Hyper-V host, the
kernel panics.

    RIP: 0010:cpuhp_issue_call+0x137/0x140
    Call Trace:
    __cpuhp_remove_state_cpuslocked+0x99/0x100
    __cpuhp_remove_state+0x1c/0x30
    hv_kexec_handler+0x23/0x30 [hv_vmbus]
    hv_machine_shutdown+0x1e/0x30
    machine_shutdown+0x10/0x20
    kernel_kexec+0x6d/0x96
    __do_sys_reboot+0x1ef/0x230
    __x64_sys_reboot+0x1d/0x20
    do_syscall_64+0x6b/0x3d8
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

This was due to hv_synic_cleanup() callback returning -EBUSY to
cpuhp_issue_call() when tearing down the VMBUS_CONNECT_CPU, even
if the vmbus_connection.conn_state = DISCONNECTED. hv_synic_cleanup()
should succeed in the case where vmbus_connection.conn_state
is DISCONNECTED.

Fix is to add an extra condition to test for
vmbus_connection.conn_state == CONNECTED on the VMBUS_CONNECT_CPU and
only return early if true. This way the kexec() path can still shut
everything down while preserving the initial behavior of preventing
CPU offlining on the VMBUS_CONNECT_CPU while the VM is running.

Fixes: 8a857c55420f29 ("Drivers: hv: vmbus: Always handle the VMBus messages on CPU0")
Signed-off-by: Chris Co <chrco@microsoft.com>
Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201110190118.15596-1-chrco@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/hv/hv.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -251,9 +251,13 @@ int hv_synic_cleanup(unsigned int cpu)
 
 	/*
 	 * Hyper-V does not provide a way to change the connect CPU once
-	 * it is set; we must prevent the connect CPU from going offline.
+	 * it is set; we must prevent the connect CPU from going offline
+	 * while the VM is running normally. But in the panic or kexec()
+	 * path where the vmbus is already disconnected, the CPU must be
+	 * allowed to shut down.
 	 */
-	if (cpu == VMBUS_CONNECT_CPU)
+	if (cpu == VMBUS_CONNECT_CPU &&
+	    vmbus_connection.conn_state == CONNECTED)
 		return -EBUSY;
 
 	/*