Re: [PATCH v2] KVM: arm64: Use BTI for nvhe

[Mostafa Saleh]

Hi Oliver,

On Fri, May 26, 2023 at 07:42:27PM +0000, Oliver Upton wrote:
> Hi Mostafa,
> 
> On Wed, May 17, 2023 at 05:35:52PM +0000, Mostafa Saleh wrote:
> > CONFIG_ARM64_BTI_KERNEL compiles the kernel to support ARMv8.5-BTI.
> > However, the nvhe code doesn't make use of it as it doesn't map any
> > pages with Guarded Page(GP) bit.
> > 
> > This patch maps nvhe(and pKVM recreated mapping of) .text section
> > with GP bit which matches the kernel handling for BTI.
> > 
> > A new flag is added to enum kvm_pgtable_prot: KVM_PGTABLE_PROT_GP_S1,
> > which represents BTI guarded page in hypervisor stage-1 page table.
> > 
> > At hyp init, SCTLR_EL2.BT is set to 1 to match EL1 configuration
> > (SCTLR_EL1.BT1) set in bti_enable().
> > 
> > hyp_init_valid_leaf_pte is added to avoid unnecessary considering GP
> > bit for stage-2.
> > 
> > Signed-off-by: Mostafa Saleh <smostafa@xxxxxxxxxx>
> > ---
> > v1 -> v2:
> > - Enable BTI for nvhe also.
> > - Only set GP bit for executable pages from pgtable code.
> > - Set SCTLR_EL2.BT when BTI is used.
> > - use system_supports_bti() for consistency.
> > - Add hyp_init_valid_leaf_pte.
> > v1: https://lore.kernel.org/all/20230516141846.792193-1-smostafa@xxxxxxxxxx/
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h |  3 +++
> >  arch/arm64/include/asm/sysreg.h      |  1 +
> >  arch/arm64/kvm/arm.c                 |  7 ++++++-
> >  arch/arm64/kvm/hyp/nvhe/hyp-init.S   |  7 +++++++
> >  arch/arm64/kvm/hyp/nvhe/setup.c      |  8 ++++++--
> >  arch/arm64/kvm/hyp/pgtable.c         | 11 ++++++++++-
> >  6 files changed, 33 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index 4cd6762bda80..5bcd06d664d3 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -151,6 +151,7 @@ enum kvm_pgtable_stage2_flags {
> >   * @KVM_PGTABLE_PROT_W:		Write permission.
> >   * @KVM_PGTABLE_PROT_R:		Read permission.
> >   * @KVM_PGTABLE_PROT_DEVICE:	Device attributes.
> > + * @KVM_PGTABLE_PROT_GP_S1:	GP(guarded page) used for BTI in stage-1 only
> >   * @KVM_PGTABLE_PROT_SW0:	Software bit 0.
> >   * @KVM_PGTABLE_PROT_SW1:	Software bit 1.
> >   * @KVM_PGTABLE_PROT_SW2:	Software bit 2.
> > @@ -163,6 +164,8 @@ enum kvm_pgtable_prot {
> >  
> >  	KVM_PGTABLE_PROT_DEVICE			= BIT(3),
> >  
> > +	KVM_PGTABLE_PROT_GP_S1			= BIT(50),
> > +
> 
> This enumeration is used to generically describe permissions that could
> be applied to either stage-1 or stage-2.
> 
> Can't we just have KVM_PGTABLE_PROT_X imply GP at hyp stage-1, assuming
> BTI is available and we're using it for the kernel?

I see it should be fine if we use GP for executable pages in EL2.
This was trying to follow the kernel in map_kernel(), where it checks
and passes the GP bit explicitly for the text section.
But nvhe use cases are much simpler and maybe we can imply that from
KVM_PGTABLE_PROT_X.

> >  	KVM_PGTABLE_PROT_SW0			= BIT(55),
> >  	KVM_PGTABLE_PROT_SW1			= BIT(56),
> >  	KVM_PGTABLE_PROT_SW2			= BIT(57),
> 
> [...]
> 
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 3d61bd3e591d..9f68e4ce6d14 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -152,6 +152,13 @@ static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
> >  	return pte;
> >  }
> >  
> > +static kvm_pte_t hyp_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
> > +{
> > +	kvm_pte_t pte = kvm_init_valid_leaf_pte(pa, attr, level);
> > +
> > +	return pte | (attr & KVM_PGTABLE_PROT_GP_S1);
> 
> This is a bit of a hack to cram the GP bit back in. I'm guessing the
> fact that our ATTR_HI mask doesn't include bit 50 led you here.
> 
> My interpretation DDI0487J D8.3.2 is that the upper attribute field is
> 63:50 for both stages of translation, but bit 50 is RES0 for stage-2.
> 
> So, rather than going this route, I'd recommend tweaking the ATTR_HI
> mask to include bit 50.
Extending ATTR_HI would simplify the code, I will update it in v3.
I was a bit hesitant as it is shared with stage-2 and BIT(50) is not
used there, but I can't see any obvious problems with this.

> > +}
> > +
> >  static kvm_pte_t kvm_init_invalid_leaf_owner(u8 owner_id)
> >  {
> >  	return FIELD_PREP(KVM_INVALID_PTE_OWNER_MASK, owner_id);
> > @@ -371,6 +378,8 @@ static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
> >  
> >  		if (device)
> >  			return -EINVAL;
> > +
> > +		attr |= prot & KVM_PGTABLE_PROT_GP_S1;
> 
> With the above suggestions, this would become:
> 
> 		if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) && system_supports_bti())
> 			attr |= KVM_PTE_LEAF_ATTR_HI_S1_GP;
> 
> >  	} else {
> >  		attr |= KVM_PTE_LEAF_ATTR_HI_S1_XN;
> >  	}
> > @@ -414,7 +423,7 @@ static bool hyp_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
> >  		return false;
> >  
> >  	data->phys += granule;
> > -	new = kvm_init_valid_leaf_pte(phys, data->attr, ctx->level);
> > +	new = hyp_init_valid_leaf_pte(phys, data->attr, ctx->level);
> >  	if (ctx->old == new)
> >  		return true;
> >  	if (!kvm_pte_valid(ctx->old))
> > -- 
> > 2.40.1.698.g37aff9b760-goog
> > 
> 
> -- 
> Thanks,
> Oliver
Thanks,
Mostafa