linux-kernel - [PATCH 5.15 677/846] xen/gntdev: fix unmap notification order

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Message-Id: <20220124184124.424938108@linuxfoundation.org>
Date:   Mon, 24 Jan 2022 19:43:14 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: [PATCH 5.15 677/846] xen/gntdev: fix unmap notification order

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>

commit ce2f46f3531a03781181b7f4bd1ff9f8c5086e7e upstream.

While working with Xen's libxenvchan library I have faced an issue with
unmap notifications sent in wrong order if both UNMAP_NOTIFY_SEND_EVENT
and UNMAP_NOTIFY_CLEAR_BYTE were requested: first we send an event channel
notification and then clear the notification byte which renders in the below
inconsistency (cli_live is the byte which was requested to be cleared on unmap):

[  444.514243] gntdev_put_map UNMAP_NOTIFY_SEND_EVENT map->notify.event 6
libxenvchan_is_open cli_live 1
[  444.515239] __unmap_grant_pages UNMAP_NOTIFY_CLEAR_BYTE at 14

Thus it is not possible to reliably implement the checks like
- wait for the notification (UNMAP_NOTIFY_SEND_EVENT)
- check the variable (UNMAP_NOTIFY_CLEAR_BYTE)
because it is possible that the variable gets checked before it is cleared
by the kernel.

To fix that we need to re-order the notifications, so the variable is first
gets cleared and then the event channel notification is sent.
With this fix I can see the correct order of execution:

[   54.522611] __unmap_grant_pages UNMAP_NOTIFY_CLEAR_BYTE at 14
[   54.537966] gntdev_put_map UNMAP_NOTIFY_SEND_EVENT map->notify.event 6
libxenvchan_is_open cli_live 0

Cc: stable@...r.kernel.org
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Link: https://lore.kernel.org/r/20211210092817.580718-1-andr2000@gmail.com
Signed-off-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/xen/gntdev.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -250,13 +250,13 @@ void gntdev_put_map(struct gntdev_priv *
 	if (!refcount_dec_and_test(&map->users))
 		return;
 
+	if (map->pages && !use_ptemod)
+		unmap_grant_pages(map, 0, map->count);
+
 	if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
 		notify_remote_via_evtchn(map->notify.event);
 		evtchn_put(map->notify.event);
 	}
-
-	if (map->pages && !use_ptemod)
-		unmap_grant_pages(map, 0, map->count);
 	gntdev_free_map(map);
 }