[PATCH v3 1/2] KVM: arm64: ptdump: Check the page tables aren't freed when accessing

[Wei-Lin Chang]

An open debugfs file keeps the KVM structure alive, but does not prevent
mmu notifier release from freeing the stage-2 page tables when the VMM’s
address space is torn down. Therefore page tables belonging to the mmus
could have been freed when a thread opens or reads the ptdump files.
Take the mmu_lock and check mmu->pgt is still alive before accessing the
page tables.

Also for the ipa_range file and stage2_levels file, switch from keeping
the pointer to the page tables to keeping the pointer to the mmu.

Fixes: 7c4f73548ed1 ("KVM: arm64: Register ptdump with debugfs on guest creation")
Reported-by: Sashiko AI <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/kvmarm/20260623144054.B91D21F000E9@xxxxxxxxxxxxxxx/
Signed-off-by: Wei-Lin Chang <weilin.chang@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/arm64/kvm/ptdump.c | 43 +++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
index 69899797dbad..4ead856c181a 100644
--- a/arch/arm64/kvm/ptdump.c
+++ b/arch/arm64/kvm/ptdump.c
@@ -115,15 +115,28 @@ static int kvm_ptdump_build_levels(struct ptdump_pg_level *level, u32 start_lvl)
 
 static struct kvm_ptdump_guest_state *kvm_ptdump_parser_create(struct kvm_s2_mmu *mmu)
 {
+	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
 	struct kvm_ptdump_guest_state *st;
-	struct kvm_pgtable *pgtable = mmu->pgt;
+	s8 start_level;
 	int ret;
 
+	/*
+	 * We only need the pgt start level to initialize the ptdump, get it
+	 * while holding the mmu_lock. It's fine if the pgt gets freed
+	 * afterwards, we'll check again when doing the actual dump.
+	 */
+	scoped_guard(write_lock, &kvm->mmu_lock) {
+		if (mmu->pgt)
+			start_level = mmu->pgt->start_level;
+		else
+			return ERR_PTR(-ENOENT);
+	}
+
 	st = kzalloc_obj(struct kvm_ptdump_guest_state, GFP_KERNEL_ACCOUNT);
 	if (!st)
 		return ERR_PTR(-ENOMEM);
 
-	ret = kvm_ptdump_build_levels(&st->level[0], pgtable->start_level);
+	ret = kvm_ptdump_build_levels(&st->level[0], start_level);
 	if (ret) {
 		kfree(st);
 		return ERR_PTR(ret);
@@ -149,6 +162,9 @@ static int kvm_ptdump_guest_show(struct seq_file *m, void *unused)
 	};
 
 	guard(write_lock)(&kvm->mmu_lock);
+	if (!mmu->pgt)
+		return 0;
+
 	st->parser_state = (struct ptdump_pg_state) {
 		.marker		= &st->ipa_marker[0],
 		.end_address	= BIT(mmu->pgt->ia_bits),
@@ -211,17 +227,27 @@ static const struct file_operations kvm_ptdump_guest_fops = {
 
 static int kvm_pgtable_range_show(struct seq_file *m, void *unused)
 {
-	struct kvm_pgtable *pgtable = m->private;
+	struct kvm_s2_mmu *mmu = m->private;
+	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
+
+	guard(write_lock)(&kvm->mmu_lock);
+
+	if (mmu->pgt)
+		seq_printf(m, "%2u\n", mmu->pgt->ia_bits);
 
-	seq_printf(m, "%2u\n", pgtable->ia_bits);
 	return 0;
 }
 
 static int kvm_pgtable_levels_show(struct seq_file *m, void *unused)
 {
-	struct kvm_pgtable *pgtable = m->private;
+	struct kvm_s2_mmu *mmu = m->private;
+	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
+
+	guard(write_lock)(&kvm->mmu_lock);
+
+	if (mmu->pgt)
+		seq_printf(m, "%1d\n", KVM_PGTABLE_MAX_LEVELS - mmu->pgt->start_level);
 
-	seq_printf(m, "%1d\n", KVM_PGTABLE_MAX_LEVELS - pgtable->start_level);
 	return 0;
 }
 
@@ -230,15 +256,12 @@ static int kvm_pgtable_debugfs_open(struct inode *m, struct file *file,
 {
 	struct kvm_s2_mmu *mmu = m->i_private;
 	struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu);
-	struct kvm_pgtable *pgtable;
 	int ret;
 
 	if (!kvm_get_kvm_safe(kvm))
 		return -ENOENT;
 
-	pgtable = mmu->pgt;
-
-	ret = single_open(file, show, pgtable);
+	ret = single_open(file, show, mmu);
 	if (ret < 0)
 		kvm_put_kvm(kvm);
 	return ret;
-- 
2.43.0