Re: [PATCH 1/1] Drivers: hv: Fix bad ref to hv_synic_eventring_tail when CPU goes offline

[Nuno Das Neves <nunodasneves@linux.microsoft.com>]

On 4/21/2025 9:31 AM, mhkelley58@gmail.com wrote:
> From: Michael Kelley <mhklinux@outlook.com>
> 
> When a CPU goes offline, hv_common_cpu_die() frees the
> hv_synic_eventring_tail memory for the CPU. But in a normal VM (i.e., not
> running in the root partition) the per-CPU memory has not been allocated,
> resulting in a bad memory reference and oops when computing the argument
> to kfree().
> 
> Fix this by freeing the memory only when running in the root partition.
> 
> Fixes: 04df7ac39943 ("Drivers: hv: Introduce per-cpu event ring tail")
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> ---
>  drivers/hv/hv_common.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index b3b11be11650..8967010db86a 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -566,9 +566,11 @@ int hv_common_cpu_die(unsigned int cpu)
>  	 * originally allocated memory is reused in hv_common_cpu_init().
>  	 */
>  
> -	synic_eventring_tail = this_cpu_ptr(hv_synic_eventring_tail);
> -	kfree(*synic_eventring_tail);
> -	*synic_eventring_tail = NULL;
> +	if (hv_root_partition()) {
> +		synic_eventring_tail = this_cpu_ptr(hv_synic_eventring_tail);
> +		kfree(*synic_eventring_tail);
> +		*synic_eventring_tail = NULL;
> +	}
>  
>  	return 0;
>  }

Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>