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

[Mikhail Kobuk]

On 08/04/2024 16:23, Danilo Krummrich wrote:
> On 4/5/24 22:05, Lyude Paul wrote:
>> 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:
> 
> Thanks for confirming.
> 
> @Mikhail, I think we should rather fix this assignment then.

Thank you all for a thorough look!

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

Considering this code bit, and the fact that fabricate_dcb_output() is 
called in drivers/gpu/drm/nouveau/nouveau_bios.c only, there's option to 
adjust function calls instead of adding BIT(or), i.e.:

fabricate_dcb_output(dcb, DCB_OUTPUT_TMDS, 0, all_heads, DCB_OUTPUT_B);

instead of current:

fabricate_dcb_output(dcb, DCB_OUTPUT_TMDS, 0, all_heads, 1);

and etc.

Should I make a new patch with adjusted calls or stick with BIT(or)?

>>>
>>> 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));
>>>>    }
>>>>    
>>>
>>