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SubjectRe: [PATCH 1/2] fbdev: Check in file_fb_info() if the fb_info was already been freed
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Hi

Am 02.05.22 um 15:09 schrieb Javier Martinez Canillas:
> If real driver probes, the fbdev core kicks out all drivers that are using
> a framebuffer that were provided by the system firmware. But it could be a
> user-space process still has a file descriptor for the fbdev device node.
>
> This can lead to a NULL pointer dereference, if the framebuffer device is
> unregistered and associated data freed, but later in the .release callback
> is attempted to access its struct fb_info.
>
> To prevent this, make file_fb_info() to also check the fb_info reference
> counter and just return NULL if this equals zero. Since that means it has
> already been freed.
>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
>
> drivers/video/fbdev/core/fbmem.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 84427470367b..20d8929df79f 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -751,8 +751,13 @@ static struct fb_info *file_fb_info(struct file *file)
> int fbidx = iminor(inode);
> struct fb_info *info = registered_fb[fbidx];
>
> - if (info != file->private_data)
> - info = NULL;
> + if (!info)
> + return NULL;
> +
> + /* check that the fb_info has not changed or was already freed */
> + if (info != file->private_data || refcount_read(&info->count) == 0)
> + return NULL;
> +

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>

However, I'm having problems with the semantics of these variables: if
we have an info from registered_fb[fbinx] and the refcount in
info->count is still 0, isn't that a consistency problem? If so, we
should print a WARN_ON().

Best regards
Thomas


> return info;
> }
>

--
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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