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

[mhkelley58@gmail.com]

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;
 }
-- 
2.25.1