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

[Javier Martinez Canillas]

Hello Thomas,

On 5/2/22 15:26, Thomas Zimmermann wrote:
> 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@xxxxxxxxxx>
>> ---
>>
>>   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@xxxxxxx>
> 
> 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().
>

That's a good point. Maybe we are being too paranoid here? If the fb_info
was set to NULL then the existing if (info != file->private_data) check
will already catch that issue.

In other words, now that fb_release() is getting the fb_info with the
file_fb_info() function instead of file->private_data directly, the NULL
pointer dereference should not happen anymore.

I think that will just drop this patch, the less we touch the fbdev code
the better IMO.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat