On Wed, May 04, 2022 at 11:57:22PM +0200, Javier Martinez Canillas wrote:
> 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.
>
> The correct thing to do is to only unregister the framebuffer in the
> driver's .remove callback, but do any cleanup in the fb_ops.fb_destroy.
>
> Suggested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
I think this should have a Fixes: line for the patch from Thomas which
changed the remove_conflicting_fb code:
27599aacbaef ("fbdev: Hot-unplug firmware fb devices on forced removal")
I think we should also mention that strictly speaking the code flow is now
wrong, because hw cleanup (like iounmap) should be done from ->remove
while sw cleanup (like calling framebuffer_release()) is the only thing
that should be done from ->fb_destroy. But the current code matches what
was happening before 27599aacbaef so more minimal "fix"
With those details added Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Same for the next patch.
-Daniel
> ---
>
> 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 94fc9c6d0411..2c198561c338 100644
> --- a/drivers/video/fbdev/simplefb.c
> +++ b/drivers/video/fbdev/simplefb.c
> @@ -84,6 +84,10 @@ 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)
> {
> struct simplefb_par *par = info->par;
> @@ -94,6 +98,8 @@ static void simplefb_destroy(struct fb_info *info)
> if (info->screen_base)
> iounmap(info->screen_base);
>
> + framebuffer_release(info);
> +
> if (mem)
> release_mem_region(mem->start, resource_size(mem));
> }
> @@ -545,8 +551,8 @@ 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
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
