linux-kernel - [PATCH 3/3] fbdev/efifb: Cleanup fb_info in .fb

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Message-Id: <20220504215806.57147-1-javierm@redhat.com>
Date:   Wed,  4 May 2022 23:58:06 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>, Peter Jones <pjones@...hat.com>,
        Javier Martinez Canillas <javierm@...hat.com>
Subject: [PATCH 3/3] fbdev/efifb: Cleanup fb_info in .fb_destroy rather than .remove

The driver is calling framebuffer_release() in its .remove callback, but
this will cause the struct fb_info to be freed too early. Since it could
be that a reference is still hold to it if user-space opened the fbdev.

This would lead to a use-after-free error if the framebuffer device was
unregistered but later a user-space process tries to close the fbdev fd.

The correct thing to do is to only unregister the framebuffer in the
driver's .remove callback, but do any cleanup in the fb_ops.fb_destroy.

Suggested-by: Daniel Vetter <daniel.vetter@...ll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
---

 drivers/video/fbdev/efifb.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
index ea42ba6445b2..cfa3dc0b4eee 100644
--- a/drivers/video/fbdev/efifb.c
+++ b/drivers/video/fbdev/efifb.c
@@ -243,6 +243,10 @@ static void efifb_show_boot_graphics(struct fb_info *info)
 static inline void efifb_show_boot_graphics(struct fb_info *info) {}
 #endif
 
+/*
+ * fb_ops.fb_destroy is called by the last put_fb_info() call at the end
+ * of unregister_framebuffer() or fb_release(). Do any cleanup here.
+ */
 static void efifb_destroy(struct fb_info *info)
 {
 	if (efifb_pci_dev)
@@ -254,6 +258,9 @@ static void efifb_destroy(struct fb_info *info)
 		else
 			memunmap(info->screen_base);
 	}
+
+	framebuffer_release(info);
+
 	if (request_mem_succeeded)
 		release_mem_region(info->apertures->ranges[0].base,
 				   info->apertures->ranges[0].size);
@@ -620,9 +627,9 @@ static int efifb_remove(struct platform_device *pdev)
 {
 	struct fb_info *info = platform_get_drvdata(pdev);
 
+	/* efifb_destroy takes care of info cleanup */
 	unregister_framebuffer(info);
 	sysfs_remove_groups(&pdev->dev.kobj, efifb_groups);
-	framebuffer_release(info);
 
 	return 0;
 }
-- 
2.35.1