Subject: Re: [PATCH] KVM: arm64: Use BTI for pKVM

From: Mostafa Saleh <[email-masked]>
Date: Wed, 17 May 2023 08:49:50 +0000

Hi Marc,

On Tue, May 16, 2023 at 04:47:10PM +0100, Marc Zyngier wrote:
> On Tue, 16 May 2023 15:18:46 +0100,
> Mostafa Saleh <[email-masked]> 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 pKVM .text section with GP bit which matches the
> > kernel handling for BTI.
> 
> Why pKVM only? Surely we can benefit from it all over the nvhe code,
> right?
Yes, I will add it also for nvhe in v2.

> > 
> > 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.
> > 
> > Signed-off-by: Mostafa Saleh <[email-masked]>
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h | 3 +++
> >  arch/arm64/kvm/hyp/nvhe/setup.c      | 8 ++++++--
> >  arch/arm64/kvm/hyp/pgtable.c         | 6 ++++--
> >  3 files changed, 13 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),
> > +
> >  	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/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
> > index 110f04627785..95f80e2b2946 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/setup.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/setup.c
> > @@ -66,7 +66,7 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
> >  {
> >  	void *start, *end, *virt = hyp_phys_to_virt(phys);
> >  	unsigned long pgt_size = hyp_s1_pgtable_pages() << PAGE_SHIFT;
> > -	enum kvm_pgtable_prot prot;
> > +	enum kvm_pgtable_prot prot = PAGE_HYP_EXEC;
> >  	int ret, i;
> >  
> >  	/* Recreate the hyp page-table using the early page allocator */
> > @@ -88,7 +88,11 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
> >  	if (ret)
> >  		return ret;
> >  
> > -	ret = pkvm_create_mappings(__hyp_text_start, __hyp_text_end, PAGE_HYP_EXEC);
> > +	/* Hypervisor text is mapped as guarded pages(GP). */
> > +	if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) && cpus_have_const_cap(ARM64_BTI))
> > +		prot |= KVM_PGTABLE_PROT_GP_S1;
> 
> Is there any reason why this isn't a final cap? I also dislike the
> IS_ENABLED(), but I can see that we don't have separate caps for
> in-kernel BTI and userspace visible BTI...
I was trying to make this close to EL1 code (system_supports_bti()),
I see in hypervisor cpus_have_const_cap is the same as cpus_have_final_cap.

Yes, I don't see a way to distinguish if BTI was enabled for the kernel
in EL2 without CONFIG_ARM64_BTI_KERNEL.

> > +
> > +	ret = pkvm_create_mappings(__hyp_text_start, __hyp_text_end, prot);
> >  	if (ret)
> >  		return ret;
> >  
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 3d61bd3e591d..028e198acd48 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -145,7 +145,8 @@ static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
> >  	u64 type = (level == KVM_PGTABLE_MAX_LEVELS - 1) ? KVM_PTE_TYPE_PAGE :
> >  							   KVM_PTE_TYPE_BLOCK;
> >  
> > -	pte |= attr & (KVM_PTE_LEAF_ATTR_LO | KVM_PTE_LEAF_ATTR_HI);
> > +	pte |= attr & (KVM_PTE_LEAF_ATTR_LO | KVM_PTE_LEAF_ATTR_HI |
> > +		       KVM_PGTABLE_PROT_GP_S1);
> >  	pte |= FIELD_PREP(KVM_PTE_TYPE, type);
> >  	pte |= KVM_PTE_VALID;
> >  
> > @@ -378,7 +379,8 @@ static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
> >  	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_AP, ap);
> >  	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_SH, sh);
> >  	attr |= KVM_PTE_LEAF_ATTR_LO_S1_AF;
> > -	attr |= prot & KVM_PTE_LEAF_ATTR_HI_SW;
> > +	attr |= prot & (KVM_PTE_LEAF_ATTR_HI_SW | KVM_PGTABLE_PROT_GP_S1);
> > +
> 
> You should probably check that the page is executable before blindly
> accepting to set the GP bit (don't accept it for non-exec pages).
Will do in v2.

> Another thing to check would be the state of SCTLR_EL2.BT, which I
> think we clear by construction, but it be worth having a look.
Yes, I see it is initialised by zero in ___kvm_hyp_init in hyp-init.S,
I believe this should be changed to 1 when BTI is enabled (as in
bti_enable() for EL1), I will update it.

Thanks,
Mostafa

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