On 10/29/21 11:22 AM, Mark Rutland wrote:

On Thu, Oct 28, 2021 at 12:36:58PM -0700, Pawan Gupta wrote:

quoted

On 28.10.2021 14:49, Mark Rutland wrote:

quoted

On Wed, Oct 27, 2021 at 06:33:22PM -0700, Pawan Gupta wrote:

quoted

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.

quoted

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.

Thanks,
Daniel

   [0] https://lore.kernel.org/bpf/YXrTev6WMXry9pFI@kroah.com/ (local)

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