[PATCH 2/2] KVM: MMU: always terminate page walks at level 1

[Paolo Bonzini]

From: Ladi Prosek <lprosek@xxxxxxxxxx>

is_last_gpte() is not equivalent to the pseudo-code given in commit
6bb69c9b69c31 ("KVM: MMU: simplify last_pte_bitmap") because an incorrect
value of last_nonleaf_level may override the result even if level == 1.

It is critical for is_last_gpte() to return true on level == 1 to
terminate page walks. Otherwise memory corruption may occur as level
is used as an index to various data structures throughout the page
walking code.  Even though the actual bug would be wherever the MMU is
initialized (as in the previous patch), be defensive and ensure here
that is_last_gpte() returns the correct value.

This patch is also enough to fix CVE-2017-12188, and suggested for
stable and distro kernels.

Fixes: 6bb69c9b69c315200ddc2bc79aee14c0184cf5b2
Cc: stable@xxxxxxxxxxxxxxx
Cc: Andy Honig <ahonig@xxxxxxxxxx>
Signed-off-by: Ladi Prosek <lprosek@xxxxxxxxxx>
[Panic if walk_addr_generic gets an incorrect level; this is a serious
 bug and it's not worth a WARN_ON where the recovery path might hide
 further exploitable issues; suggested by Andrew Honig. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/mmu.c         | 14 +++++++-------
 arch/x86/kvm/paging_tmpl.h |  3 ++-
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 3c25f20115bc..7a69cf053711 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3974,19 +3974,19 @@ static inline bool is_last_gpte(struct kvm_mmu *mmu,
 				unsigned level, unsigned gpte)
 {
 	/*
-	 * PT_PAGE_TABLE_LEVEL always terminates.  The RHS has bit 7 set
-	 * iff level <= PT_PAGE_TABLE_LEVEL, which for our purpose means
-	 * level == PT_PAGE_TABLE_LEVEL; set PT_PAGE_SIZE_MASK in gpte then.
-	 */
-	gpte |= level - PT_PAGE_TABLE_LEVEL - 1;
-
-	/*
 	 * The RHS has bit 7 set iff level < mmu->last_nonleaf_level.
 	 * If it is clear, there are no large pages at this level, so clear
 	 * PT_PAGE_SIZE_MASK in gpte if that is the case.
 	 */
 	gpte &= level - mmu->last_nonleaf_level;
 
+	/*
+	 * PT_PAGE_TABLE_LEVEL always terminates.  The RHS has bit 7 set
+	 * iff level <= PT_PAGE_TABLE_LEVEL, which for our purpose means
+	 * level == PT_PAGE_TABLE_LEVEL; set PT_PAGE_SIZE_MASK in gpte then.
+	 */
+	gpte |= level - PT_PAGE_TABLE_LEVEL - 1;
+
 	return gpte & PT_PAGE_SIZE_MASK;
 }
 
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 86b68dc5a649..f18d1f8d332b 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -334,10 +334,11 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 		--walker->level;
 
 		index = PT_INDEX(addr, walker->level);
-
 		table_gfn = gpte_to_gfn(pte);
 		offset    = index * sizeof(pt_element_t);
 		pte_gpa   = gfn_to_gpa(table_gfn) + offset;
+
+		BUG_ON(walker->level < 1);
 		walker->table_gfn[walker->level - 1] = table_gfn;
 		walker->pte_gpa[walker->level - 1] = pte_gpa;
 
-- 
1.8.3.1