[PATCH] KVM: arm64: Adjust range correctly during host stage-2 faults

[Quentin Perret <qperret@google.com>]

host_stage2_adjust_range() tries to find the largest block mapping that
fits within a memory or mmio region (represented by a kvm_mem_range in
this function) during host stage-2 faults under pKVM. To do so, it walks
the host stage-2 page-table, finds the faulting PTE and its level, and
then progressively increments the level until it finds a granule of the
appropriate size. However, the condition in the loop implementing the
above is broken as it checks kvm_level_supports_block_mapping() for the
next level instead of the current, so pKVM may attempt to map a region
larger than can be covered with a single block.

This is not a security problem and is quite rare in practice (the
kvm_mem_range check usually forces host_stage2_adjust_range() to choose a
smaller granule), but this is clearly not the expected behaviour.

Refactor the loop to fix the bug and improve readability.

Fixes: c4f0935e4d95 ("KVM: arm64: Optimize host memory aborts")
Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 95d7534c9679..8957734d6183 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -479,6 +479,7 @@ static int host_stage2_adjust_range(u64 addr, struct kvm_mem_range *range)
 {
 	struct kvm_mem_range cur;
 	kvm_pte_t pte;
+	u64 granule;
 	s8 level;
 	int ret;
 
@@ -496,18 +497,21 @@ static int host_stage2_adjust_range(u64 addr, struct kvm_mem_range *range)
 		return -EPERM;
 	}
 
-	do {
-		u64 granule = kvm_granule_size(level);
+	for (; level <= KVM_PGTABLE_LAST_LEVEL; level++) {
+		if (!kvm_level_supports_block_mapping(level))
+			continue;
+		granule = kvm_granule_size(level);
 		cur.start = ALIGN_DOWN(addr, granule);
 		cur.end = cur.start + granule;
-		level++;
-	} while ((level <= KVM_PGTABLE_LAST_LEVEL) &&
-			!(kvm_level_supports_block_mapping(level) &&
-			  range_included(&cur, range)));
+		if (!range_included(&cur, range))
+			continue;
+		*range = cur;
+		return 0;
+	}
 
-	*range = cur;
+	WARN_ON(1);
 
-	return 0;
+	return -EINVAL;
 }
 
 int host_stage2_idmap_locked(phys_addr_t addr, u64 size,
-- 
2.50.0.714.g196bf9f422-goog