[PATCH] drm/i915/gvt: Fix life cycle reference on KVM mm

[Zhenyu Wang]

Handle guest mm access life cycle properly with mmget()/mmput()
through guest init()/exit(). As noted by Linus, use_mm() depends
on valid live page table but KVM's mmgrab() doesn't guarantee that.
As vGPU usage depends on guest VM life cycle, need to make sure to
use mmget()/mmput() to guarantee VM address access.

Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Zhi Wang <zhi.a.wang at intel.com>
Reviewed-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw at linux.intel.com>
---
 drivers/gpu/drm/i915/gvt/kvmgt.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index 71751be329e3..4a0988747d08 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -32,6 +32,7 @@
 #include <linux/device.h>
 #include <linux/mm.h>
 #include <linux/mmu_context.h>
+#include <linux/sched/mm.h>
 #include <linux/types.h>
 #include <linux/list.h>
 #include <linux/rbtree.h>
@@ -1614,9 +1615,16 @@ static int kvmgt_guest_init(struct mdev_device *mdev)
 	if (__kvmgt_vgpu_exist(vgpu, kvm))
 		return -EEXIST;
 
+	if (!mmget_not_zero(kvm->mm)) {
+		gvt_vgpu_err("Can't get KVM mm reference\n");
+		return -EINVAL;
+	}
+
 	info = vzalloc(sizeof(struct kvmgt_guest_info));
-	if (!info)
+	if (!info) {
+		mmput(kvm->mm);
 		return -ENOMEM;
+	}
 
 	vgpu->handle = (unsigned long)info;
 	info->vgpu = vgpu;
@@ -1647,6 +1655,8 @@ static bool kvmgt_guest_exit(struct kvmgt_guest_info *info)
 	debugfs_remove(info->debugfs_cache_entries);
 
 	kvm_page_track_unregister_notifier(info->kvm, &info->track_node);
+	if (info->kvm->mm)
+		mmput(info->kvm->mm);
 	kvm_put_kvm(info->kvm);
 	kvmgt_protect_table_destroy(info);
 	gvt_cache_destroy(info->vgpu);
-- 
2.18.0