[05/13] media: mgb4: protect driver against spectre

Frequency range is set from sysfs via frequency_range_store(),
being vulnerable to spectre, as reported by smatch:

	drivers/media/pci/mgb4/mgb4_cmt.c:231 mgb4_cmt_set_vin_freq_range() warn: potential spectre issue 'cmt_vals_in' [r]
	drivers/media/pci/mgb4/mgb4_cmt.c:238 mgb4_cmt_set_vin_freq_range() warn: possible spectre second half.  'reg_set'

Fix it.

Fixes: 0ab13674a9bd ("media: pci: mgb4: Added Digiteq Automotive MGB4 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
---
 drivers/media/pci/mgb4/mgb4_cmt.c | 2 ++
 1 file changed, 2 insertions(+)

Em Wed, 16 Oct 2024 13:59:18 +0200
Martin Tůma <tumic@gpxsee.org> escreveu:

> On 16. 10. 24 12:22 odp., Mauro Carvalho Chehab wrote:
> > Frequency range is set from sysfs via frequency_range_store(),
> > being vulnerable to spectre, as reported by smatch:
> > 
> > 	drivers/media/pci/mgb4/mgb4_cmt.c:231 mgb4_cmt_set_vin_freq_range() warn: potential spectre issue 'cmt_vals_in' [r]
> > 	drivers/media/pci/mgb4/mgb4_cmt.c:238 mgb4_cmt_set_vin_freq_range() warn: possible spectre second half.  'reg_set'
> > 
> > Fix it.
> > 
> > Fixes: 0ab13674a9bd ("media: pci: mgb4: Added Digiteq Automotive MGB4 driver")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> > ---
> >   drivers/media/pci/mgb4/mgb4_cmt.c | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/media/pci/mgb4/mgb4_cmt.c b/drivers/media/pci/mgb4/mgb4_cmt.c
> > index 70dc78ef193c..a25b68403bc6 100644
> > --- a/drivers/media/pci/mgb4/mgb4_cmt.c
> > +++ b/drivers/media/pci/mgb4/mgb4_cmt.c
> > @@ -227,6 +227,8 @@ void mgb4_cmt_set_vin_freq_range(struct mgb4_vin_dev *vindev,
> >   	u32 config;
> >   	size_t i;
> >   
> > +	freq_range = array_index_nospec(freq_range, ARRAY_SIZE(cmt_vals_in));
> > +
> >   	addr = cmt_addrs_in[vindev->config->id];
> >   	reg_set = cmt_vals_in[freq_range];
> >     
> 
> I still do not fully understand the exact vulnerability here, but the 
> patch should definitely not do any harm, so I'm ok with it even if it's 
> real purpose would only be to silence the smatch warning :-)

With Spectre, just checking if freq_range is between 0 and the
size of the array is not enough, as malicious code could use CPU
speculative logic to retrieve data from memory outside the limits
of the array.

As freq_range is specified by the user via sysfs attribute
frequency_range, it is subject to Spectre v1 attack as described
at Documentation/admin-guide/hw-vuln/spectre.rst. 

Silencing smatch is a plus.

> 
> Reviewed-by: Martin Tůma <martin.tuma@digiteqautomotive.com>

Thanks!

Thanks,
Mauro

Hi,

On 18. 10. 24 6:32 dop., Mauro Carvalho Chehab wrote:
> Em Wed, 16 Oct 2024 13:59:18 +0200
> Martin Tůma <tumic@gpxsee.org> escreveu:
> 
>> On 16. 10. 24 12:22 odp., Mauro Carvalho Chehab wrote:
>>> Frequency range is set from sysfs via frequency_range_store(),
>>> being vulnerable to spectre, as reported by smatch:
>>>
>>> 	drivers/media/pci/mgb4/mgb4_cmt.c:231 mgb4_cmt_set_vin_freq_range() warn: potential spectre issue 'cmt_vals_in' [r]
>>> 	drivers/media/pci/mgb4/mgb4_cmt.c:238 mgb4_cmt_set_vin_freq_range() warn: possible spectre second half.  'reg_set'
>>>
>>> Fix it.
>>>
>>> Fixes: 0ab13674a9bd ("media: pci: mgb4: Added Digiteq Automotive MGB4 driver")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
>>> ---
>>>    drivers/media/pci/mgb4/mgb4_cmt.c | 2 ++
>>>    1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/media/pci/mgb4/mgb4_cmt.c b/drivers/media/pci/mgb4/mgb4_cmt.c
>>> index 70dc78ef193c..a25b68403bc6 100644
>>> --- a/drivers/media/pci/mgb4/mgb4_cmt.c
>>> +++ b/drivers/media/pci/mgb4/mgb4_cmt.c
>>> @@ -227,6 +227,8 @@ void mgb4_cmt_set_vin_freq_range(struct mgb4_vin_dev *vindev,
>>>    	u32 config;
>>>    	size_t i;
>>>    
>>> +	freq_range = array_index_nospec(freq_range, ARRAY_SIZE(cmt_vals_in));
>>> +
>>>    	addr = cmt_addrs_in[vindev->config->id];
>>>    	reg_set = cmt_vals_in[freq_range];
>>>      
>>
>> I still do not fully understand the exact vulnerability here, but the
>> patch should definitely not do any harm, so I'm ok with it even if it's
>> real purpose would only be to silence the smatch warning :-)
> 
> With Spectre, just checking if freq_range is between 0 and the
> size of the array is not enough, as malicious code could use CPU
> speculative logic to retrieve data from memory outside the limits
> of the array.
> 
> As freq_range is specified by the user via sysfs attribute
> frequency_range, it is subject to Spectre v1 attack as described
> at Documentation/admin-guide/hw-vuln/spectre.rst.
> 

I do know the general theory about the "spectre bounds check 
fix/workaround", what I was referring is this part in the documentation:

"Such speculative memory accesses can leave side effects, creating side 
channels which leak information to the attacker."

I do not see/understand the exact "information leak" that could happen 
here on this particular place. But as already stated in the original 
answer, I don't have to understand everything ;-)

M.

> Silencing smatch is a plus.
> 
>>
>> Reviewed-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
> 
> Thanks!
> 
> Thanks,
> Mauro
>