Re: [PATCH v4 10/13] KVM: arm64: nVHE: Support CONFIG_CFI_CLANG at EL2

From: Will Deacon <will@kernel.org>
To: "Pierre-Clément Tosi" <ptosi@google.com>
Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Vincent Donnefort <vdonnefort@google.com>
Subject: Re: [PATCH v4 10/13] KVM: arm64: nVHE: Support CONFIG_CFI_CLANG at EL2
Date: Wed, 5 Jun 2024 17:11:37 +0100	[thread overview]
Message-ID: <20240605161137.GC22199@willie-the-truck> (raw)
In-Reply-To: <ucvvmwiur2qhagm4tcsbffftzaktthqnryi74k5ajtolhx4qor@via5txfexwlc>

On Tue, Jun 04, 2024 at 05:04:40PM +0100, Pierre-Clément Tosi wrote:
> On Mon, Jun 03, 2024 at 03:45:30PM +0100, Will Deacon wrote:
> > On Wed, May 29, 2024 at 01:12:16PM +0100, Pierre-Clément Tosi wrote:
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-init.S b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> > > index d859c4de06b6..b1c8977e2812 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> > > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> > > @@ -5,6 +5,7 @@
> > >   */
> > >  
> > >  #include <linux/arm-smccc.h>
> > > +#include <linux/cfi_types.h>
> > >  #include <linux/linkage.h>
> > >  
> > >  #include <asm/alternative.h>
> > > @@ -267,8 +268,11 @@ SYM_CODE_END(__kvm_handle_stub_hvc)
> > >  
> > >  /*
> > >   * void __pkvm_init_switch_pgd(phys_addr_t pgd, void *sp, void (*fn)(void));
> > > + *
> > > + * SYM_TYPED_FUNC_START() allows C to call this ID-mapped function indirectly
> > > + * using a physical pointer without triggering a kCFI failure.
> > >   */
> > > -SYM_FUNC_START(__pkvm_init_switch_pgd)
> > > +SYM_TYPED_FUNC_START(__pkvm_init_switch_pgd)
> > >  	/* Turn the MMU off */
> > >  	pre_disable_mmu_workaround
> > >  	mrs	x9, sctlr_el2
> > 
> > I still think this last hunk should be merged with the earlier patch
> > fixing up the prototype of __pkvm_init_switch_pgd().
> 
> Unfortunately, this is not possible because
> 
>   SYM_TYPED_FUNC_START(__pkvm_init_switch_pgd)
> 
> makes the assembler generate an unresolved symbol for the function type, which
> the compiler only generates (from the C declaration) if it compiles with kCFI so
> that moving this hunk to an earlier patch results in a linker error:
> 
>   ld.lld: error: undefined symbol: __kvm_nvhe___kcfi_typeid___pkvm_init_switch_pgd
> 
> OTOH, moving it to a later patch triggers a kCFI (runtime) panic.
> 
> As a result, this hunk *must* be part of this patch.

Argh, thanks for the explanation. I thought CONFIG_CFI_CLANG would save
us here, but that's already enabled for the rest of the kernel so now I
understand what you mean.

> > With that:
> > 
> > Acked-by: Will Deacon <will@kernel.org>
> 
> As I haven't followed your suggestion, I'll ignore this.

You can keep the Ack.

Cheers,

Will

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