Re: [RFC PATCH] arm64: fix the missing ktpi= cmdline check in arm64_kernel_unmapped_at_el0()

[Will Deacon]

On Tue, Mar 17, 2020 at 01:57:19PM +0000, Mark Brown wrote:
> On Tue, Mar 17, 2020 at 12:43:24PM +0000, Will Deacon wrote:
> > On Tue, Mar 17, 2020 at 12:10:51PM +0000, Mark Rutland wrote:
> > > On Tue, Mar 17, 2020 at 07:47:08PM +0800, Hongbo Yao wrote:
> 
> > > > Kpti cannot be disabled from the kernel cmdline after the commit
> > > > 09e3c22a("arm64: Use a variable to store non-global mappings decision").
> 
> > > > Bring back the missing check of kpti= command-line option to fix the
> > > > case where the SPE driver complains the missing "kpti-off" even it has
> > > > already been set.
> 
> > > > -	return arm64_use_ng_mappings;
> > > > +	return arm64_use_ng_mappings &&
> > > > +		cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0);
> > > >  }
> 
> > This probably isn't the right fix, since this will mean that early mappings
> > will be global and we'll have to go through the painful page-table rewrite
> > logic when the cap gets enabled for KASLR-enabled kernels.
> 
> Aren't we looking for a rewrite from non-global to global here (disable
> KPTI where we would otherwise have it), which we don't currently have
> code for?

What I mean is that cpus_have_const_cap() will be false initially, so we'll
put down global mappings early on because PTE_MAYBE_NG will be 0, which
means that we'll have to invoke the rewriting code if we then realise we
want non-global mappings after the caps are finalised.

> > Maybe a better bodge is something like:
> 
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 0b6715625cf6..ad10f55b7bb9 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1085,6 +1085,8 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> >  		if (!__kpti_forced) {
> >  			str = "KASLR";
> >  			__kpti_forced = 1;
> > +		} else if (__kpti_forced < 0) {
> > +			arm64_use_ng_mappings = false;
> >  		}
> >  	}
> 
> That is probably a good idea but I think that runs too late to affect
> the early mappings, they're done based on kaslr_requires_kpti() well
> before we start secondaries.  My first pass not having paged everything
> back in yet is that there needs to be command line parsing in
> kaslr_requires_kpti() but as things stand the command line isn't
> actually ready then...

Yeah, and I think you probably run into chicken and egg problems mapping
the thing. With the change above, it's true that /some/ mappings will
still be nG if you pass kpti=off, but I was hoping that didn't really matter
:)

What was the behaviour prior to your patch? If it used to work without
any nG mappings, then I suppose we should try to restore that behaviour.

Will