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

[Danilo Krummrich <dakr@redhat.com>]

On 4/10/24 17:39, Mikhail Kobuk wrote:
> 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)?

Please send a new patch adjusting the calls using enum nouveau_or, that
seems to be cleaner.

- Danilo

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