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

[Will Deacon]

On Tue, Mar 17, 2020 at 12:10:51PM +0000, Mark Rutland wrote:
> [Adding Catalin and LAKML]
> 
> Mark.
> 
> 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.
> > 
> > Signed-off-by: Hongbo Yao <yaohongbo@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/mmu.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> > index 3c9533322558..ebbc0d3ac2f7 100644
> > --- a/arch/arm64/include/asm/mmu.h
> > +++ b/arch/arm64/include/asm/mmu.h
> > @@ -34,7 +34,8 @@ extern bool arm64_use_ng_mappings;
> >  
> >  static inline bool arm64_kernel_unmapped_at_el0(void)
> >  {
> > -	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.

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;
 		}
 	}
 

Will