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 <redacted>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <redacted>
---

(no changes since v1)

  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;
  }