[PATCH] Drivers: hv: vmbus: Allow cleanup of VMBUS_CONNECT_CPU if disconnected

[PATCH] Drivers: hv: vmbus: Allow cleanup of VMBUS_CONNECT_CPU if disconnected

[Chris Co]

From: Chris Co <chrco@microsoft.com>

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>
Cc: stable@vger.kernel.org
---
 drivers/hv/hv.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 0cde10fe0e71..f202ac7f4b3d 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -244,9 +244,13 @@ 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;
 
 	/*
-- 
2.17.1

Re: [PATCH] Drivers: hv: vmbus: Allow cleanup of VMBUS_CONNECT_CPU if disconnected

[Andrea Parri]

On Tue, Nov 10, 2020 at 07:01:18PM +0000, Chris Co wrote:
> From: Chris Co <chrco@microsoft.com>
> 
> 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>
> Cc: stable@vger.kernel.org

Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>

Thanks,
  Andrea


> ---
>  drivers/hv/hv.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 0cde10fe0e71..f202ac7f4b3d 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -244,9 +244,13 @@ 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;
>  
>  	/*
> -- 
> 2.17.1
> 

Re: [PATCH] Drivers: hv: vmbus: Allow cleanup of VMBUS_CONNECT_CPU if disconnected

[Wei Liu]

On Tue, Nov 10, 2020 at 09:18:33PM +0100, Andrea Parri wrote:
> On Tue, Nov 10, 2020 at 07:01:18PM +0000, Chris Co wrote:
> > From: Chris Co <chrco@microsoft.com>
> > 
> > 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>
> > Cc: stable@vger.kernel.org
> 
> Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>

Applied to hyperv-fixes. Thanks Chris and Andrea.

Wei.