Re: [PATCH v2 1/2] arch/Kconfig: Make CONFIG_CPU_SPECTRE available for all architectures

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On 29.10.2021 18:05, Daniel Borkmann wrote:
>On 10/29/21 11:22 AM, Mark Rutland wrote:
>>On Thu, Oct 28, 2021 at 12:36:58PM -0700, Pawan Gupta wrote:
>>>On 28.10.2021 14:49, Mark Rutland wrote:
>>>>On Wed, Oct 27, 2021 at 06:33:22PM -0700, Pawan Gupta wrote:
>>>>>Borrow CONFIG_CPU_SPECTRE from ARM to be available for all
>>>>>architectures. This will help in configuration of features that depend
>>>>>on CPU being affected by spectre class of vulnerabilities.
>>>>>
>>>>>Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>>>>
>>>>Given that spectre isn't one specific issue, biut rather a blanket term
>>>>for a bunch of things that can have variable overlap, I don't think this
>>>>makes much sense unless we're going to add finer-grained options for all
>>>>the variants, and IMO it'd make more sene for the architectures to
>>>>directly select the things that'd otherwise be dependent on this.
>>>
>>>Isn't ARM already using CPU_SPECTRE for selecting things:
>>>
>>>	config HARDEN_BRANCH_PREDICTOR
>>>	     bool "Harden the branch predictor against aliasing attacks" if EXPERT
>>>	     depends on CPU_SPECTRE
>>
>>It's true that arch/arm does, but that's not true for other
>>architectures, e.g. powerpc or arm64, and and as above I don't think it
>>makes sense to make this generic in its current form because "spectre"
>>is a somewhat vague generic term.
>>
>>>This was the whole motivation for doing the same for x86.
>>>
>>>Adding a condition for all architectures is also okay, but its going to
>>>a little messier:
>>>
>>>	 config BPF_UNPRIV_DEFAULT_OFF
>>>	        default y if X86 || ARM || ...
>>>
>>>This approach would make sense if architectures wants to explicitly
>>>select the defaults irrespective of architecture being affected by
>>>spectre.
>>
>>If we're going to change the default for some architectures, I think
>>it'd make much more sense to just do that for all, without any
>>arch-specific conditionality, i.e.
>>
>>	config BPF_UNPRIV_DEFAULT_OFF
>>		default y
>
>Lets just go with 'default y'. The main rationale for this change was motivated
>by spectre, so would have been good to indicate this also with an explicit
>dependency for broken HW, not just help description. Pretty much agreeing with
>Greg here [0]. Eventually, we might need some arch generic way to determine arch-
>common spectre type bugs, so that for unaffected HW we don't need to apply some
>of them from verifier, but that's still tbd.

I will send a patch soon with 'default y'.

Thanks,
Pawan