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
>