Re: [PATCH] drm: nv04: Add check to avoid out of bounds access

[Lyude Paul <lyude@redhat.com>]

On Fri, 2024-04-05 at 17:53 +0200, Danilo Krummrich wrote:
> On 3/31/24 08:45, Mikhail Kobuk wrote:
> > Output Resource (dcb->or) value is not guaranteed to be non-zero
> > (i.e.
> > in drivers/gpu/drm/nouveau/nouveau_bios.c, in
> > 'fabricate_dcb_encoder_table()'
> > 'dcb->or' is assigned value '0' in call to
> > 'fabricate_dcb_output()').
> 
> I don't really know much about the semantics of this code.
> 
> Looking at fabricate_dcb_output() though I wonder if the intention
> was to assign
> BIT(or) to entry->or.
> 
> @Lyude, can you help here?

This code is definitely a bit before my time as well - but I think
you're completely correct. Especially considering this bit I found in
nouveau_bios.h:

enum nouveau_or {
	DCB_OUTPUT_A = (1 << 0),
	DCB_OUTPUT_B = (1 << 1),
	DCB_OUTPUT_C = (1 << 2)
};


> 
> Otherwise, for parsing the DCB entries, it seems that the bound
> checks are
> happening in olddcb_outp_foreach() [1].
> 
> [1]
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nouveau_bios.c#L1331
> 
> > 
> > Add check to validate 'dcb->or' before it's used.
> > 
> > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > 
> > Fixes: 2e5702aff395 ("drm/nouveau: fabricate DCB encoder table for
> > iMac G4")
> > Signed-off-by: Mikhail Kobuk <m.kobuk@ispras.ru>
> > ---
> >   drivers/gpu/drm/nouveau/dispnv04/dac.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/nouveau/dispnv04/dac.c
> > b/drivers/gpu/drm/nouveau/dispnv04/dac.c
> > index d6b8e0cce2ac..0c8d4fc95ff3 100644
> > --- a/drivers/gpu/drm/nouveau/dispnv04/dac.c
> > +++ b/drivers/gpu/drm/nouveau/dispnv04/dac.c
> > @@ -428,7 +428,7 @@ void nv04_dac_update_dacclk(struct drm_encoder
> > *encoder, bool enable)
> >   	struct drm_device *dev = encoder->dev;
> >   	struct dcb_output *dcb = nouveau_encoder(encoder)->dcb;
> >   
> > -	if (nv_gf4_disp_arch(dev)) {
> > +	if (nv_gf4_disp_arch(dev) && ffs(dcb->or)) {
> >   		uint32_t *dac_users = &nv04_display(dev)-
> > >dac_users[ffs(dcb->or) - 1];
> >   		int dacclk_off = NV_PRAMDAC_DACCLK +
> > nv04_dac_output_offset(encoder);
> >   		uint32_t dacclk = NVReadRAMDAC(dev, 0,
> > dacclk_off);
> > @@ -453,7 +453,7 @@ bool nv04_dac_in_use(struct drm_encoder
> > *encoder)
> >   	struct drm_device *dev = encoder->dev;
> >   	struct dcb_output *dcb = nouveau_encoder(encoder)->dcb;
> >   
> > -	return nv_gf4_disp_arch(encoder->dev) &&
> > +	return nv_gf4_disp_arch(encoder->dev) && ffs(dcb->or) &&
> >   		(nv04_display(dev)->dac_users[ffs(dcb->or) - 1] &
> > ~(1 << dcb->index));
> >   }
> >   
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat