[Devel] [PATCH RHEL8 COMMIT] proc connector: protect from task cpu migration in send_msg_ve

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-305.3.1.vz8.7.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-305.3.1.vz8.7.11
------>
commit b52c9d8a3f42108bd8a36abee89193b93a18facb
Author: Konstantin Khorenko <khorenko at virtuozzo.com>
Date:   Thu Sep 9 23:18:47 2021 +0300

    proc connector: protect from task cpu migration in send_msg_ve
    
    this_cpu_ptr() should be used under local_lock() (or task migration
    should be disabled instead), so switch the order.
    
    Fixes: 0e60020d2a ("proc connector: take number of listeners and per-cpu
    conters from VE")
    
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
    Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn at virtuozzo.com>
---
 drivers/connector/cn_proc.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/connector/cn_proc.c b/drivers/connector/cn_proc.c
index 9b63792f0dff..ce7b33345db5 100644
--- a/drivers/connector/cn_proc.c
+++ b/drivers/connector/cn_proc.c
@@ -63,9 +63,15 @@ static inline void send_msg_ve(struct ve_struct *ve, struct cn_msg *msg)
 {
 	struct local_event *le_ptr;
 
-	le_ptr = this_cpu_ptr(ve->cn->local_event);
-	local_lock(&le_ptr->lock);
+	/*
+	 * The following hack with local_event->lock address works only
+	 * till the "lock" is the first field in the local_event struct,
+	 * so be of the safe side.
+	 */
+	BUILD_BUG_ON(offsetof(struct local_event, lock) != 0);
+	local_lock(&ve->cn->local_event->lock);
 
+	le_ptr = this_cpu_ptr(ve->cn->local_event);
 	msg->seq = le_ptr->count++;
 	((struct proc_event *)msg->data)->cpu = smp_processor_id();
 
@@ -77,7 +83,7 @@ static inline void send_msg_ve(struct ve_struct *ve, struct cn_msg *msg)
 	 */
 	cn_netlink_send_ve(ve, msg, 0, CN_IDX_PROC, GFP_NOWAIT);
 
-	local_unlock(&le_ptr->lock);
+	local_unlock(&ve->cn->local_event->lock);
 }
 
 static struct cn_msg *cn_msg_fill(__u8 *buffer, struct ve_struct *ve,