Re: [PATCH v2] fbdev: Fix invalid page access after closing deferred I/O devices

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 29.01.23 um 09:28 schrieb Takashi Iwai:
> When a fbdev with deferred I/O is once opened and closed, the dirty
> pages still remain queued in the pageref list, and eventually later
> those may be processed in the delayed work.  This may lead to a
> corruption of pages, hitting an Oops.

Do you have more information on this problem?

The mmap'ed buffer of the fbdev device comes from a vmalloc call. That 
memory's location never changes; even across pairs of open/close on the 
device file. I'm surprised that a page entry becomes invalid.

In drm_fbdev_cleanup(), we first remove the fbdefio at [1] and then 
vfree() the shadow buffer. So the memory should still be around until 
fbdevio is gone.

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_fb_helper.c#L2146

> 
> This patch makes sure to cancel the delayed work and clean up the
> pageref list at closing the device for addressing the bug.  A part of
> the cleanup code is factored out as a new helper function that is
> called from the common fb_release().

The delayed work is required to copy the framebuffer to the device 
output. So if it's just canceled, could this result in missing updates?

There's a call to cancel_delayed_work_sync() in the new helper 
fb_deferred_io_release(). Is this the right function? Maybe 
flush_delayed_work() is a better choice.

> 
> Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>

This could use a Fixes tag. It's not exactly clear to me when this 
problem got originally introduced, but the recent refactoring seems a 
candidate.

Fixes: 56c134f7f1b5 ("fbdev: Track deferred-I/O pages in pageref struct")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Zack Rusin <zackr@vmware.com>
Cc: VMware Graphics Reviewers <linux-graphics-maintainer@vmware.com>
Cc: Jaya Kumar <jayalk@intworks.biz>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Cc: Bernie Thompson <bernie@plugable.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Stephen Kitt <steve@sk2.org>
Cc: Peter Suti <peter.suti@streamunlimited.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: ye xingchen <ye.xingchen@zte.com.cn>
Cc: Petr Mladek <pmladek@suse.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Tom Rix <trix@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Cc: linux-hyperv@vger.kernel.org
Cc: <stable@vger.kernel.org> # v5.19+

> ---
> v1->v2: Fix build error without CONFIG_FB_DEFERRED_IO
> 
>   drivers/video/fbdev/core/fb_defio.c | 10 +++++++++-
>   drivers/video/fbdev/core/fbmem.c    |  4 ++++
>   include/linux/fb.h                  |  1 +
>   3 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
> index c730253ab85c..583cbcf09446 100644
> --- a/drivers/video/fbdev/core/fb_defio.c
> +++ b/drivers/video/fbdev/core/fb_defio.c
> @@ -313,7 +313,7 @@ void fb_deferred_io_open(struct fb_info *info,
>   }
>   EXPORT_SYMBOL_GPL(fb_deferred_io_open);
>   
> -void fb_deferred_io_cleanup(struct fb_info *info)
> +void fb_deferred_io_release(struct fb_info *info)
>   {
>   	struct fb_deferred_io *fbdefio = info->fbdefio;
>   	struct page *page;
> @@ -327,6 +327,14 @@ void fb_deferred_io_cleanup(struct fb_info *info)
>   		page = fb_deferred_io_page(info, i);
>   		page->mapping = NULL;
>   	}
> +}
> +EXPORT_SYMBOL_GPL(fb_deferred_io_release);

It's all in the same module. No need to export this symbol.

Best regards
Thomas

> +
> +void fb_deferred_io_cleanup(struct fb_info *info)
> +{
> +	struct fb_deferred_io *fbdefio = info->fbdefio;
> +
> +	fb_deferred_io_release(info);
>   
>   	kvfree(info->pagerefs);
>   	mutex_destroy(&fbdefio->lock);
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 3a6c8458eb8d..ab3545a00abc 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1454,6 +1454,10 @@ __releases(&info->lock)
>   	struct fb_info * const info = file->private_data;
>   
>   	lock_fb_info(info);
> +#if IS_ENABLED(CONFIG_FB_DEFERRED_IO)
> +	if (info->fbdefio)
> +		fb_deferred_io_release(info);
> +#endif
>   	if (info->fbops->fb_release)
>   		info->fbops->fb_release(info,1);


>   	module_put(info->fbops->owner);
> diff --git a/include/linux/fb.h b/include/linux/fb.h
> index 96b96323e9cb..73eb1f85ea8e 100644
> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -662,6 +662,7 @@ extern int  fb_deferred_io_init(struct fb_info *info);
>   extern void fb_deferred_io_open(struct fb_info *info,
>   				struct inode *inode,
>   				struct file *file);
> +extern void fb_deferred_io_release(struct fb_info *info);
>   extern void fb_deferred_io_cleanup(struct fb_info *info);
>   extern int fb_deferred_io_fsync(struct file *file, loff_t start,
>   				loff_t end, int datasync);

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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