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[v2,17/26] KVM: x86/mmu: Pass access information to make_huge_page_split_spte()

Message ID 20220311002528.2230172-18-dmatlack@google.com (mailing list archive)
State New, archived
Headers show
Series Extend Eager Page Splitting to the shadow MMU | expand

Commit Message

David Matlack March 11, 2022, 12:25 a.m. UTC 
Currently make_huge_page_split_spte() assumes execute permissions can be
granted to any 4K SPTE when splitting huge pages. This is true for the
TDP MMU but is not necessarily true for the shadow MMU. Huge pages
mapped by the shadow MMU may be shadowing huge pages that the guest has
disallowed execute permissions.

No functional change intended.

Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 arch/x86/kvm/mmu/spte.c    | 5 +++--
 arch/x86/kvm/mmu/spte.h    | 3 ++-
 arch/x86/kvm/mmu/tdp_mmu.c | 2 +-
 3 files changed, 6 insertions(+), 4 deletions(-)

Comments

Peter Xu March 16, 2022, 8:44 a.m. UTC | #1 
On Fri, Mar 11, 2022 at 12:25:19AM +0000, David Matlack wrote:
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 85b7bc333302..541b145b2df2 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1430,7 +1430,7 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
>  	 * not been linked in yet and thus is not reachable from any other CPU.
>  	 */
>  	for (i = 0; i < PT64_ENT_PER_PAGE; i++)
> -		sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i);
> +		sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i, ACC_ALL);

Pure question: is it possible that huge_spte is RO while we passed in
ACC_ALL here (which has the write bit set)?  Would it be better if we make
it a "bool exec" to be clearer?
David Matlack March 22, 2022, 11:08 p.m. UTC | #2 
On Wed, Mar 16, 2022 at 1:44 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Mar 11, 2022 at 12:25:19AM +0000, David Matlack wrote:
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 85b7bc333302..541b145b2df2 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1430,7 +1430,7 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
> >        * not been linked in yet and thus is not reachable from any other CPU.
> >        */
> >       for (i = 0; i < PT64_ENT_PER_PAGE; i++)
> > -             sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i);
> > +             sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i, ACC_ALL);
>
> Pure question: is it possible that huge_spte is RO while we passed in
> ACC_ALL here (which has the write bit set)?

Yes that is possible, but only if KVM the page is RO due to host-side
policies (e.g. RO memslot or RO VMA). "access" here is the
guest-allowed access permissions, similar to the pte_access parameter
to mmu_set_spte(). e.g. notice how the TDP MMU passes ACC_ALL to
make_spte().

> Would it be better if we make it a "bool exec" to be clearer?

But all that being said, the ACC_ALL stuff is confusing for exactly
the reason you pointed out so it doesn't make sense to duplicate it
further. I agree it would make more sense to pass in bool exec.

>
> --
> Peter Xu
>
diff mbox series

Patch

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index d10189d9c877..7294f95464a7 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -216,7 +216,8 @@  static u64 make_spte_executable(u64 spte)
  * This is used during huge page splitting to build the SPTEs that make up the
  * new page table.
  */
-u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index)
+u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index,
+			      unsigned int access)
 {
 	u64 child_spte;
 	int child_level;
@@ -244,7 +245,7 @@  u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index)
 		 * When splitting to a 4K page, mark the page executable as the
 		 * NX hugepage mitigation no longer applies.
 		 */
-		if (is_nx_huge_page_enabled())
+		if (is_nx_huge_page_enabled() && (access & ACC_EXEC_MASK))
 			child_spte = make_spte_executable(child_spte);
 	}
 
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 73f12615416f..c7ccdd5c440d 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -415,7 +415,8 @@  bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
 	       u64 old_spte, bool prefetch, bool can_unsync,
 	       bool host_writable, u64 *new_spte);
-u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index);
+u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index,
+			      unsigned int access);
 u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled);
 u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access);
 u64 mark_spte_for_access_track(u64 spte);
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 85b7bc333302..541b145b2df2 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1430,7 +1430,7 @@  static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
 	 * not been linked in yet and thus is not reachable from any other CPU.
 	 */
 	for (i = 0; i < PT64_ENT_PER_PAGE; i++)
-		sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i);
+		sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i, ACC_ALL);
 
 	/*
 	 * Replace the huge spte with a pointer to the populated lower level