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

[Danilo Krummrich]

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?

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 at 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));
>   }
>