linux-kernel - [PATCH AUTOSEL 5.15 06/17] Revert "fbdev: Make fb_release() return -ENODEV if fbdev was unregistered"

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Message-Id: <20220518122753.342758-6-sashal@kernel.org>
Date:   Wed, 18 May 2022 08:27:40 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Sasha Levin <sashal@...nel.org>, daniel@...ll.ch,
        deller@....de, sam@...nborg.org, tzimmermann@...e.de,
        deng.changcheng@....com.cn, xiyuyang19@...an.edu.cn,
        zheyuma97@...il.com, alexander.deucher@....com,
        penguin-kernel@...ove.sakura.ne.jp, thunder.leizhen@...wei.com,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: [PATCH AUTOSEL 5.15 06/17] Revert "fbdev: Make fb_release() return -ENODEV if fbdev was unregistered"

From: Javier Martinez Canillas <javierm@...hat.com>

[ Upstream commit 135332f34ba2662bc1e32b5c612e06a8cc41a053 ]

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504115917.758787-1-javierm@redhat.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 8e38a7a5cf2f..0371ad233fdf 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1436,10 +1436,7 @@ 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