Re: [PATCH 1/2] KVM: X86: Check pte present first in __shadow_walk_next()

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 12, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> So far, the loop bodies already ensure the pte is present before calling
> __shadow_walk_next().  But checking pte present in __shadow_walk_next()
> is a more prudent way of programing and loop bodies will not need
> to always check it.  It allows us removing is_shadow_present_pte()
> in the loop bodies.

There needs to be more analysis in the changelog, as there are many more callers
to __shadow_walk_next() than the three that are modified in the next patch.  It
might even make sense to squash the two patches together, i.e. make it a "move"
instead of an "add + remove", and then explicitly explain why it's ok to add the
check for paths that do _not_ currently have a !is_shadow_present_pte() in the
loop body.

Specifically, FNAME(fetch) via shadow_walk_next() and __direct_map() via
for_each_shadow_entry() do not currently terminate their walks with a !PRESENT,
but they get away with it because they install present non-leaf SPTEs in the loop
itself.

The other argument for the audit (changelog+patch) of all users is that the next
patch misses FNAME(invlpg), e.g. 

@@ -977,7 +980,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
                        FNAME(update_pte)(vcpu, sp, sptep, &gpte);
                }

-               if (!is_shadow_present_pte(*sptep) || !sp->unsync_children)
+               if (!sp->unsync_children)
                        break;
        }
        write_unlock(&vcpu->kvm->mmu_lock);

It would also be worthwhile to document via the changelog that terminating on
!is_shadow_present_pte() is 100% the correct behavior, as walking past a !PRESENT
SPTE would lead to attempting to read a the next level SPTE from a garbage
iter->shadow_addr.

And for clarity and safety, I think it would be worth adding the patch below as
a prep patch to document and enforce that walking the non-leaf SPTEs when faulting
in a page should never terminate early.


From 1c202a7e82b1931e4eb37b23aa9d7108340a6cd2 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Thu, 12 Aug 2021 08:38:35 -0700
Subject: [PATCH] KVM: x86/mmu: Verify shadow walk doesn't terminate early in
 page faults

WARN and bail if the shadow walk for faulting in a SPTE terminates early,
i.e. doesn't reach the expected level because the walk encountered a
terminal SPTE.  The shadow walks for page faults are subtle in that they
install non-leaf SPTEs (zapping leaf SPTEs if necessary!) in the loop
body, and consume the newly created non-leaf SPTE in the loop control,
e.g. __shadow_walk_next().  In other words, the walks guarantee that the
walk will stop if and only if the target level is reached by installing
non-leaf SPTEs to guarantee the walk remains valid.

Opportunistically use fault->goal-level instead of it.level in
FNAME(fetch) to further clarify that KVM always installs the leaf SPTE at
the target level.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c         | 3 +++
 arch/x86/kvm/mmu/paging_tmpl.h | 7 +++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a272ccbddfa1..2a243ae1d64c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2992,6 +2992,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 			account_huge_nx_page(vcpu->kvm, sp);
 	}

+	if (WARN_ON_ONCE(it.level != fault->goal_level))
+		return -EFAULT;
+
 	ret = mmu_set_spte(vcpu, it.sptep, ACC_ALL,
 			   fault->write, fault->goal_level, base_gfn, fault->pfn,
 			   fault->prefault, fault->map_writable);
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index f70afecbf3a2..3a8a7b2f9979 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -749,9 +749,12 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 		}
 	}

+	if (WARN_ON_ONCE(it.level != fault->goal_level))
+		return -EFAULT;
+
 	ret = mmu_set_spte(vcpu, it.sptep, gw->pte_access, fault->write,
-			   it.level, base_gfn, fault->pfn, fault->prefault,
-			   fault->map_writable);
+			   fault->goal_level, base_gfn, fault->pfn,
+			   fault->prefault, fault->map_writable);
 	if (ret == RET_PF_SPURIOUS)
 		return ret;

--
2.33.0.rc1.237.g0d66db33f3-goog