linux-kernel - [PATCH 1/2] fbdev: Check in file_fb_info() if the fb

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Message-Id: <20220502130944.363776-2-javierm@redhat.com>
Date:   Mon,  2 May 2022 15:09:43 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Maxime Ripard <maxime@...no.tech>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        Sam Ravnborg <sam@...nborg.org>,
        Xiyu Yang <xiyuyang19@...an.edu.cn>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: [PATCH 1/2] fbdev: Check in file_fb_info() if the fb_info was already been freed

If real driver probes, the fbdev core kicks out all drivers that are using
a framebuffer that were provided by the system firmware. But it could be a
user-space process still has a file descriptor for the fbdev device node.

This can lead to a NULL pointer dereference, if the framebuffer device is
unregistered and associated data freed, but later in the .release callback
is attempted to access its struct fb_info.

To prevent this, make file_fb_info() to also check the fb_info reference
counter and just return NULL if this equals zero. Since that means it has
already been freed.

Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
---

 drivers/video/fbdev/core/fbmem.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 84427470367b..20d8929df79f 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -751,8 +751,13 @@ static struct fb_info *file_fb_info(struct file *file)
 	int fbidx = iminor(inode);
 	struct fb_info *info = registered_fb[fbidx];
 
-	if (info != file->private_data)
-		info = NULL;
+	if (!info)
+		return NULL;
+
+	/* check that the fb_info has not changed or was already freed */
+	if (info != file->private_data || refcount_read(&info->count) == 0)
+		return NULL;
+
 	return info;
 }
 
-- 
2.35.1