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Subject[PATCH 5.10 136/137] xen-netback: take a reference to the RX task thread
Date
From: Roger Pau Monne <roger.pau@citrix.com>

commit 107866a8eb0b664675a260f1ba0655010fac1e08 upstream.

Do this in order to prevent the task from being freed if the thread
returns (which can be triggered by the frontend) before the call to
kthread_stop done as part of the backend tear down. Not taking the
reference will lead to a use-after-free in that scenario. Such
reference was taken before but dropped as part of the rework done in
2ac061ce97f4.

Reintroduce the reference taking and add a comment this time
explaining why it's needed.

This is XSA-374 / CVE-2021-28691.

Fixes: 2ac061ce97f4 ('xen/netback: cleanup init and deinit code')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/xen-netback/interface.c | 6 ++++++
1 file changed, 6 insertions(+)

--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -685,6 +685,7 @@ static void xenvif_disconnect_queue(stru
{
if (queue->task) {
kthread_stop(queue->task);
+ put_task_struct(queue->task);
queue->task = NULL;
}

@@ -745,6 +746,11 @@ int xenvif_connect_data(struct xenvif_qu
if (IS_ERR(task))
goto kthread_err;
queue->task = task;
+ /*
+ * Take a reference to the task in order to prevent it from being freed
+ * if the thread function returns before kthread_stop is called.
+ */
+ get_task_struct(task);

task = kthread_run(xenvif_dealloc_kthread, queue,
"%s-dealloc", queue->name);

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