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From:   Javier Martinez Canillas <javierm@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Helge Deller <deller@gmx.de>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Thomas Zimmermann <tzimmermann@suse.de>
Subject: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb_release()
Date:   Fri,  6 May 2022 00:04:13 +0200
Message-Id: <20220505220413.365977-1-javierm@redhat.com>
In-Reply-To: <20220505215947.364694-1-javierm@redhat.com>
References: <20220505215947.364694-1-javierm@redhat.com>
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From: Daniel Vetter <daniel.vetter@ffwll.ch>

Most fbdev drivers have issues with the fb_info lifetime, because call to
framebuffer_release() from their driver's .remove callback, rather than
doing from fbops.fb_destroy callback.

Doing that will destroy the fb_info too early, while references to it may
still exist, leading to a use-after-free error.

To prevent this, check the fb_info reference counter when attempting to
kfree the data structure in framebuffer_release(). That will leak it but
at least will prevent the mentioned error.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
---

(no changes since v1)

 drivers/video/fbdev/core/fbsysfs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
index 8c1ee9ecec3d..c2a60b187467 100644
--- a/drivers/video/fbdev/core/fbsysfs.c
+++ b/drivers/video/fbdev/core/fbsysfs.c
@@ -80,6 +80,10 @@ void framebuffer_release(struct fb_info *info)
 {
 	if (!info)
 		return;
+
+	if (WARN_ON(refcount_read(&info->count)))
+		return;
+
 	kfree(info->apertures);
 	kfree(info);
 }
-- 
2.35.1