releases/4.9.137/xen-blkfront-avoid-null-blkfront_info-dereference-on-device-removal.patch

From f92898e7f32e3533bfd95be174044bc349d416ca Mon Sep 17 00:00:00 2001
From: Vasilis Liaskovitis <vliaskovitis@suse.com>
Date: Mon, 15 Oct 2018 15:25:08 +0200
Subject: xen/blkfront: avoid NULL blkfront_info dereference on device removal
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From: Vasilis Liaskovitis <vliaskovitis@suse.com>

commit f92898e7f32e3533bfd95be174044bc349d416ca upstream.

If a block device is hot-added when we are out of grants,
gnttab_grant_foreign_access fails with -ENOSPC (log message "28
granting access to ring page") in this code path:

  talk_to_blkback ->
	setup_blkring ->
		xenbus_grant_ring ->
			gnttab_grant_foreign_access

and the failing path in talk_to_blkback sets the driver_data to NULL:

 destroy_blkring:
        blkif_free(info, 0);

        mutex_lock(&blkfront_mutex);
        free_info(info);
        mutex_unlock(&blkfront_mutex);

        dev_set_drvdata(&dev->dev, NULL);

This results in a NULL pointer BUG when blkfront_remove and blkif_free
try to access the failing device's NULL struct blkfront_info.

Cc: stable@vger.kernel.org # 4.5 and later
Signed-off-by: Vasilis Liaskovitis <vliaskovitis@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/block/xen-blkfront.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -2524,6 +2524,9 @@ static int blkfront_remove(struct xenbus
 
 	dev_dbg(&xbdev->dev, "%s removed", xbdev->nodename);
 
+	if (!info)
+		return 0;
+
 	blkif_free(info, 0);
 
 	mutex_lock(&info->mutex);