linux-kernel - [PATCH v2] Revert "fbdev: Make fb_release() return -ENODEV if fbdev was unregistered"

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Message-Id: <20220504115917.758787-1-javierm@redhat.com>
Date:   Wed,  4 May 2022 13:59:17 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Maxime Ripard <maxime@...no.tech>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: [PATCH v2] Revert "fbdev: Make fb_release() return -ENODEV if fbdev was unregistered"

This reverts commit aafa025c76dcc7d1a8c8f0bdefcbe4eb480b2f6a. That commit
attempted to fix a NULL pointer dereference, caused by the struct fb_info
associated with a framebuffer device to not longer be valid when the file
descriptor was closed.

The issue was exposed by commit 27599aacbaef ("fbdev: Hot-unplug firmware
fb devices on forced removal"), which added a new path that goes through
the struct device removal instead of directly unregistering the fb.

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. This meant that due to this switch,
the fb_info was now destroyed too early, while references still existed,
while before it was simply leaked.

The patch we're reverting here reinstated that leak, hence "fixed" the
regression. But the proper solution is to fix the drivers to not release
the fb_info too soon.

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

Changes in v2:
- Add more info in the commit message about why it's crashing and how
  the reverted commit was papering over the issue (Daniel Vetter).
- Add Daniel Vetter's Reviewed-by tag.

 drivers/video/fbdev/core/fbmem.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 97eb0dee411c..a6bb0e438216 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1434,10 +1434,7 @@ fb_release(struct inode *inode, struct file *file)
 __acquires(&info->lock)
 __releases(&info->lock)
 {
-	struct fb_info * const info = file_fb_info(file);
-
-	if (!info)
-		return -ENODEV;
+	struct fb_info * const info = file->private_data;

 	lock_fb_info(info);
 	if (info->fbops->fb_release)
-- 
2.35.1