Re: [PATCH 6/9] KVM: x86/mmu: Bug the VM if a vCPU ends up in long mode without PAE enabled

[David Matlack <dmatlack@google.com>]

On Thu, May 11, 2023 at 04:59:14PM -0700, Sean Christopherson wrote:
> Promote the ASSERT(), which is quite dead code in KVM, into a KVM_BUG_ON()
> for KVM's sanity check that CR4.PAE=1 if the vCPU is in long mode when
> performing a walk of guest page tables.  The sanity is quite cheap since
> neither EFER nor CR4.PAE requires a VMREAD, especially relative to the
> cost of walking the guest page tables.
> 
> More importantly, the sanity check would have prevented the true badness
> fixed by commit 112e66017bff ("KVM: nVMX: add missing consistency checks
> for CR0 and CR4").  The missed consistency check resulted in some versions
> of KVM corrupting the on-stack guest_walker structure due to KVM thinking
> there are 4/5 levels of page tables, but wiring up the MMU hooks to point
> at the paging32 implementation, which only allocates space for two levels
> of page tables in "struct guest_walker32".
> 
> Queue a page fault for injection if the assertion fails, as the sole
> caller, FNAME(gva_to_gpa), assumes that walker.fault contains sane info

FNAME(page_fault)->FNAME(walk_addr)->FNAME(walk_addr_generic) is another
caller but I think the same reasoning here applies.

> on a walk failure, i.e. avoid making the situation worse between the time
> the assertion fails and when KVM kicks the vCPU out to userspace (because
> the VM is bugged).
> 
> Move the check below the initialization of "pte_access" so that the
> aforementioned to-be-injected page fault doesn't consume uninitialized
> stack data.  The information _shouldn't_ reach the guest or userspace,
> but there's zero downside to being paranoid in this case.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/mmu/paging_tmpl.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index a3fc7c1a7f8d..f297e9311dcd 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -338,7 +338,6 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
>  	}
>  #endif
>  	walker->max_level = walker->level;
> -	ASSERT(!(is_long_mode(vcpu) && !is_pae(vcpu)));
>  
>  	/*
>  	 * FIXME: on Intel processors, loads of the PDPTE registers for PAE paging
> @@ -348,6 +347,10 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
>  	nested_access = (have_ad ? PFERR_WRITE_MASK : 0) | PFERR_USER_MASK;
>  
>  	pte_access = ~0;
> +
> +	if (KVM_BUG_ON(is_long_mode(vcpu) && !is_pae(vcpu), vcpu->kvm))
> +		goto error;

This if() deserves a comment since it's queueing a page fault for what
is likely a KVM bug. As a reader that'd be pretty jarring to see.

> +
>  	++walker->level;
>  
>  	do {
> -- 
> 2.40.1.606.ga4b1b128d6-goog
>