Drivers: hv: Fix bad ref to hv_synic_eventring_tail when CPU goes offline

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: 04df7ac ("Drivers: hv: Introduce per-cpu event ring tail")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250421163134.2024-1-mhklinux@outlook.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250421163134.2024-1-mhklinux@outlook.com>

Author: mhklinux

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;
 }