linux-kernel - [PATCH 5.15 010/102] fbdev: simplefb: Cleanup fb_info in .fb

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Date:   Mon, 16 May 2022 21:35:44 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Daniel Vetter <daniel.vetter@...ll.ch>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 010/102] fbdev: simplefb: Cleanup fb_info in .fb_destroy rather than .remove

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

[ Upstream commit 666b90b3ce9e4aac1e1deba266c3a230fb3913b0 ]

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.

To prevent this, move the framebuffer_release() call to fb_ops.fb_destroy
instead of doing it in the driver's .remove callback.

Strictly speaking, the code flow in the driver is still wrong because all
the hardware cleanupd (i.e: iounmap) should be done in .remove while the
software cleanup (i.e: releasing the framebuffer) should be done in the
.fb_destroy handler. But this at least makes to match the behavior before
commit 27599aacbaef ("fbdev: Hot-unplug firmware fb devices on forced removal").

Fixes: 27599aacbaef ("fbdev: Hot-unplug firmware fb devices on forced removal")
Suggested-by: Daniel Vetter <daniel.vetter@...ll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220505220456.366090-1-javierm@redhat.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/video/fbdev/simplefb.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/simplefb.c b/drivers/video/fbdev/simplefb.c
index b63074fd892e..a2e3a4690025 100644
--- a/drivers/video/fbdev/simplefb.c
+++ b/drivers/video/fbdev/simplefb.c
@@ -70,12 +70,18 @@ struct simplefb_par;
 static void simplefb_clocks_destroy(struct simplefb_par *par);
 static void simplefb_regulators_destroy(struct simplefb_par *par);
 
+/*
+ * 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 simplefb_destroy(struct fb_info *info)
 {
 	simplefb_regulators_destroy(info->par);
 	simplefb_clocks_destroy(info->par);
 	if (info->screen_base)
 		iounmap(info->screen_base);
+
+	framebuffer_release(info);
 }
 
 static const struct fb_ops simplefb_ops = {
@@ -520,8 +526,8 @@ static int simplefb_remove(struct platform_device *pdev)
 {
 	struct fb_info *info = platform_get_drvdata(pdev);
 
+	/* simplefb_destroy takes care of info cleanup */
 	unregister_framebuffer(info);
-	framebuffer_release(info);
 
 	return 0;
 }
-- 
2.35.1