Re: [PATCH 4.4 28/31] video: hgafb: fix potential NULL pointer dereference

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, May 25, 2021 at 06:06:15PM -0300, Igor Torrente wrote:
> Hi Pavel,
> 
> On 5/25/21 5:47 PM, Pavel Machek wrote:
> > Hi!
> > 
> > > From: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
> > > 
> > > commit dc13cac4862cc68ec74348a80b6942532b7735fa upstream.
> > > 
> > > The return of ioremap if not checked, and can lead to a NULL to be
> > > assigned to hga_vram. Potentially leading to a NULL pointer
> > > dereference.
> > > 
> > > The fix adds code to deal with this case in the error label and
> > > changes how the hgafb_probe handles the return of hga_card_detect.
> > 
> > This will break hgafb completely, right? And crash system without hga
> > card as a bonus.
> > 
> > > +++ b/drivers/video/fbdev/hgafb.c
> > > @@ -285,6 +285,8 @@ static int hga_card_detect(void)
> > >   	hga_vram_len  = 0x08000;
> > >   	hga_vram = ioremap(0xb0000, hga_vram_len);
> > > +	if (!hga_vram)
> > > +		return -ENOMEM;
> > >   	if (request_region(0x3b0, 12, "hgafb"))
> > >   		release_io_ports = 1;
> > > @@ -344,13 +346,18 @@ static int hga_card_detect(void)
> > >   			hga_type_name = "Hercules";
> > >   			break;
> > >   	}
> > > -	return 1;
> > > +	return 0;
> > 
> > Ok, so calling convention is now "0 means detected".
> > 
> > 
> > > @@ -548,13 +555,11 @@ static struct fb_ops hgafb_ops = {
> > >   static int hgafb_probe(struct platform_device *pdev)
> > >   {
> > >   	struct fb_info *info;
> > > +	int ret;
> > ...
> > > +	ret = hga_card_detect();
> > > +	if (!ret)
> > > +		return ret;
> > >   	printk(KERN_INFO "hgafb: %s with %ldK of memory detected.\n",
> > >   		hga_type_name, hga_vram_len/1024);
> > > 
> > 
> > If the card is detected, 0 is returned, !0 is true, and we abort
> > detection....
> 
> Yes, you are right! There's a patch that fixes it:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git/commit/?h=char-misc-linus&id=02625c965239b71869326dd0461615f27307ecb3
> 
> As far as I know, this patch should be queue up soon to all stable branches.
> 
> Greg should have more details about it.

Good catch, I'll go queue that up now, thanks.

greg k-h