[1/3] KVM: arm64: Fix memory leak on stage2 update of a valid PTE

Commit Message

Marc Zyngier Dec. 4, 2020, 6:19 p.m. UTC

From: Yanan Wang <[e-mail]>

When installing a new leaf PTE onto an invalid ptep, we need to
get_page(ptep) to account for the new mapping.

However, simply updating a valid PTE shouldn't result in any
additional refcounting, as there is new mapping. This otherwise
results in a page being forever wasted.

Address this by fixing-up the refcount in stage2_map_walker_try_leaf()
if the PTE was already valid, balancing out the later get_page()
in stage2_map_walk_leaf().

Signed-off-by: Yanan Wang <[e-mail]>
[maz: update commit message, add comment in the code]
Signed-off-by: Marc Zyngier <[e-mail]>
Acked-by: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
---
 arch/arm64/kvm/hyp/pgtable.c | 9 +++++++++
 1 file changed, 9 insertions(+)

Patch

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 0271b4a3b9fe..2beba1dc40ec 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -470,6 +470,15 @@  static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
 	if (!kvm_block_mapping_supported(addr, end, phys, level))
 		return false;
 
+	/*
+	 * If the PTE was already valid, drop the refcount on the table
+	 * early, as it will be bumped-up again in stage2_map_walk_leaf().
+	 * This ensures that the refcount stays constant across a valid to
+	 * valid PTE update.
+	 */
+	if (kvm_pte_valid(*ptep))
+		put_page(virt_to_page(ptep));
+
 	if (kvm_set_valid_leaf_pte(ptep, phys, data->attr, level))
 		goto out;