linux-kernel - [PATCH] fbdev: vesafb: Fix a use-after-free due early fb

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Message-Id: <20220526194752.307192-1-javierm@redhat.com>
Date:   Thu, 26 May 2022 21:47:52 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Pascal Ernster <dri-devel@...dfalcon.net>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: [PATCH] fbdev: vesafb: Fix a use-after-free due early fb_info cleanup

Commit b3c9a924aab6 ("fbdev: vesafb: Cleanup fb_info in .fb_destroy rather
than .remove") fixed a use-after-free error due the vesafb driver freeing
the fb_info in the .remove handler instead of doing it in .fb_destroy.

This can happen if the .fb_destroy callback is executed after the .remove
callback, since the former tries to access a pointer freed by the latter.

But that change didn't take into account that another possible scenario is
that .fb_destroy is called before the .remove callback. For example, if no
process has the fbdev chardev opened by the time the driver is removed.

If that's the case, fb_info will be freed when unregister_framebuffer() is
called, making the fb_info pointer accessed in vesafb_remove() after that
to no longer be valid.

To prevent that, move the expression containing the info->par to happen
before the unregister_framebuffer() function call.

Fixes: b3c9a924aab6 ("fbdev: vesafb: Cleanup fb_info in .fb_destroy rather than .remove")
Reported-by: Pascal Ernster <dri-devel@...dfalcon.net>
Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
---

 drivers/video/fbdev/vesafb.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/vesafb.c b/drivers/video/fbdev/vesafb.c
index e25e8de5ff67..929d4775cb4b 100644
--- a/drivers/video/fbdev/vesafb.c
+++ b/drivers/video/fbdev/vesafb.c
@@ -490,11 +490,12 @@ static int vesafb_remove(struct platform_device *pdev)
 {
 	struct fb_info *info = platform_get_drvdata(pdev);

-	/* vesafb_destroy takes care of info cleanup */
-	unregister_framebuffer(info);
 	if (((struct vesafb_par *)(info->par))->region)
 		release_region(0x3c0, 32);

+	/* vesafb_destroy takes care of info cleanup */
+	unregister_framebuffer(info);
+
 	return 0;
 }

-- 
2.36.1