linux-kernel - Re: [PATCH 1/1] Drivers: hv: Fix bad ref to hv_synic_eventring

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Message-ID: <495d5444-b82e-4ec7-9095-d34f0ac8d40c@linux.microsoft.com>
Date: Wed, 23 Apr 2025 10:50:27 -0700
From: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To: mhklinux@...look.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com, kys@...rosoft.com, linux-kernel@...r.kernel.org,
 linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 1/1] Drivers: hv: Fix bad ref to hv_synic_eventring_tail
 when CPU goes offline

On 4/21/2025 9:31 AM, mhkelley58@...il.com wrote:
> From: Michael Kelley <mhklinux@...look.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@...look.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@...ux.microsoft.com>