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From:   Lv Yunlong <lyl2019@mail.ustc.edu.cn>
To:     oleksandr_andrushchenko@epam.com, airlied@linux.ie, daniel@ffwll.ch
Cc:     dri-devel@lists.freedesktop.org, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Subject: [PATCH] gpu/xen: Fix a use after free in xen_drm_drv_init
Date:   Mon, 22 Mar 2021 18:46:56 -0700
Message-Id: <20210323014656.10068-1-lyl2019@mail.ustc.edu.cn>
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In function displback_changed, has the call chain
displback_connect(front_info)->xen_drm_drv_init(front_info).
We can see that drm_info is assigned to front_info->drm_info
and drm_info is freed in fail branch in xen_drm_drv_init().

Later displback_disconnect(front_info) is called and it calls
xen_drm_drv_fini(front_info) cause a use after free by
drm_info = front_info->drm_info statement.

My patch has done two things. First fixes the fail label which
drm_info = kzalloc() failed and still free the drm_info.
Second sets front_info->drm_info to NULL to avoid uaf.

Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
---
 drivers/gpu/drm/xen/xen_drm_front.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xen/xen_drm_front.c b/drivers/gpu/drm/xen/xen_drm_front.c
index 30d9adf31c84..9f14d99c763c 100644
--- a/drivers/gpu/drm/xen/xen_drm_front.c
+++ b/drivers/gpu/drm/xen/xen_drm_front.c
@@ -521,7 +521,7 @@ static int xen_drm_drv_init(struct xen_drm_front_info *front_info)
 	drm_dev = drm_dev_alloc(&xen_drm_driver, dev);
 	if (IS_ERR(drm_dev)) {
 		ret = PTR_ERR(drm_dev);
-		goto fail;
+		goto fail_dev;
 	}
 
 	drm_info->drm_dev = drm_dev;
@@ -551,8 +551,10 @@ static int xen_drm_drv_init(struct xen_drm_front_info *front_info)
 	drm_kms_helper_poll_fini(drm_dev);
 	drm_mode_config_cleanup(drm_dev);
 	drm_dev_put(drm_dev);
-fail:
+fail_dev:
 	kfree(drm_info);
+	front_info->drm_info = NULL;
+fail:
 	return ret;
 }
 
-- 
2.25.1