[PATCH v4 16/19] KVM: Ensure validity of memslot with respect to kvm_get_dirty_log()

[Sean Christopherson <sean.j.christopherson@intel.com>]

Rework kvm_get_dirty_log() so that it "returns" the associated memslot
on success.  A future patch will rework memslot handling such that
id_to_memslot() can return NULL, returning the memslot makes it more
obvious that the validity of the memslot has been verified, i.e.
precludes the need to add validity checks in the arch code that are
technically unnecessary.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/powerpc/kvm/book3s_pr.c |  6 +-----
 arch/s390/kvm/kvm-s390.c     | 12 ++----------
 include/linux/kvm_host.h     |  2 +-
 virt/kvm/kvm_main.c          | 27 +++++++++++++++++++--------
 4 files changed, 23 insertions(+), 24 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
index d2ee00bc7077..485ca134a949 100644
--- a/arch/powerpc/kvm/book3s_pr.c
+++ b/arch/powerpc/kvm/book3s_pr.c
@@ -1897,7 +1897,6 @@ static int kvmppc_vcpu_run_pr(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
 static int kvm_vm_ioctl_get_dirty_log_pr(struct kvm *kvm,
 					 struct kvm_dirty_log *log)
 {
-	struct kvm_memslots *slots;
 	struct kvm_memory_slot *memslot;
 	struct kvm_vcpu *vcpu;
 	ulong ga, ga_end;
@@ -1907,15 +1906,12 @@ static int kvm_vm_ioctl_get_dirty_log_pr(struct kvm *kvm,
 
 	mutex_lock(&kvm->slots_lock);
 
-	r = kvm_get_dirty_log(kvm, log, &is_dirty);
+	r = kvm_get_dirty_log(kvm, log, &is_dirty, &memslot);
 	if (r)
 		goto out;
 
 	/* If nothing is dirty, don't bother messing with page tables. */
 	if (is_dirty) {
-		slots = kvm_memslots(kvm);
-		memslot = id_to_memslot(slots, log->slot);
-
 		ga = memslot->base_gfn << PAGE_SHIFT;
 		ga_end = ga + (memslot->npages << PAGE_SHIFT);
 
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 9e38973fd2cc..b0f5a3b7cb01 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -610,9 +610,8 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 {
 	int r;
 	unsigned long n;
-	struct kvm_memslots *slots;
 	struct kvm_memory_slot *memslot;
-	int is_dirty = 0;
+	int is_dirty;
 
 	if (kvm_is_ucontrol(kvm))
 		return -EINVAL;
@@ -623,14 +622,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 	if (log->slot >= KVM_USER_MEM_SLOTS)
 		goto out;
 
-	slots = kvm_memslots(kvm);
-	memslot = id_to_memslot(slots, log->slot);
-	r = -ENOENT;
-	if (!memslot->dirty_bitmap)
-		goto out;
-
-	kvm_arch_sync_dirty_log(kvm, memslot);
-	r = kvm_get_dirty_log(kvm, log, &is_dirty);
+	r = kvm_get_dirty_log(kvm, log, &is_dirty, &memslot);
 	if (r)
 		goto out;
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index eb54d196c0cb..7d666eedd203 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -828,7 +828,7 @@ void kvm_arch_dirty_log_tlb_flush(struct kvm *kvm,
 #else /* !CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT */
 int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log);
 int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
-		      int *is_dirty);
+		      int *is_dirty, struct kvm_memory_slot **memslot);
 #endif
 
 int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_level,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4a4b5339f229..999a2a0c83f5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1202,31 +1202,42 @@ static int kvm_vm_ioctl_set_memory_region(struct kvm *kvm,
 }
 
 #ifndef CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT
-int kvm_get_dirty_log(struct kvm *kvm,
-			struct kvm_dirty_log *log, int *is_dirty)
+/**
+ * kvm_get_dirty_log - get a snapshot of dirty pages
+ * @kvm:	pointer to kvm instance
+ * @log:	slot id and address to which we copy the log
+ * @is_dirty:	set to '1' if any dirty pages were found
+ * @memslot:	set to the associated memslot, always valid on success
+ */
+int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
+		      int *is_dirty, struct kvm_memory_slot **memslot)
 {
 	struct kvm_memslots *slots;
-	struct kvm_memory_slot *memslot;
 	int i, as_id, id;
 	unsigned long n;
 	unsigned long any = 0;
 
+	*memslot = NULL;
+	*is_dirty = 0;
+
 	as_id = log->slot >> 16;
 	id = (u16)log->slot;
 	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
 		return -EINVAL;
 
 	slots = __kvm_memslots(kvm, as_id);
-	memslot = id_to_memslot(slots, id);
-	if (!memslot->dirty_bitmap)
+	*memslot = id_to_memslot(slots, id);
+	if (!(*memslot)->dirty_bitmap)
 		return -ENOENT;
 
-	n = kvm_dirty_bitmap_bytes(memslot);
+	kvm_arch_sync_dirty_log(kvm, *memslot);
+
+	n = kvm_dirty_bitmap_bytes(*memslot);
 
 	for (i = 0; !any && i < n/sizeof(long); ++i)
-		any = memslot->dirty_bitmap[i];
+		any = (*memslot)->dirty_bitmap[i];
 
-	if (copy_to_user(log->dirty_bitmap, memslot->dirty_bitmap, n))
+	if (copy_to_user(log->dirty_bitmap, (*memslot)->dirty_bitmap, n))
 		return -EFAULT;
 
 	if (any)
-- 
2.24.1