Re: [PATCH 09/27] drm/i915/gvt: Protect gfn hash table with dedicated mutex

[Yan Zhao]

On Fri, Dec 23, 2022 at 12:57:21AM +0000, Sean Christopherson wrote:
> Add and use a new mutex, gfn_lock, to protect accesses to the hash table
> used to track which gfns are write-protected when shadowing the guest's
> GTT.  This fixes a bug where kvmgt_page_track_write(), which doesn't hold
> kvm->mmu_lock, could race with intel_gvt_page_track_remove() and trigger
> a use-after-free.
> 
> Fixing kvmgt_page_track_write() by taking kvm->mmu_lock is not an option
> as mmu_lock is a r/w spinlock, and intel_vgpu_page_track_handler() might
> sleep when acquiring vgpu->cache_lock deep down the callstack:
> 
>   intel_vgpu_page_track_handler()
>   |
>   |->  page_track->handler / ppgtt_write_protection_handler()
>        |
>        |-> ppgtt_handle_guest_write_page_table_bytes()
>            |
>            |->  ppgtt_handle_guest_write_page_table()
>                 |
>                 |-> ppgtt_handle_guest_entry_removal()
>                     |
>                     |-> ppgtt_invalidate_pte()
>                         |
>                         |-> intel_gvt_dma_unmap_guest_page()
>                             |
>                             |-> mutex_lock(&vgpu->cache_lock);
> 
This gfn_lock could lead to deadlock in below sequence.

(1) kvm_write_track_add_gfn() to GFN 1
(2) kvmgt_page_track_write() for GFN 1
kvmgt_page_track_write()
|
|->mutex_lock(&info->vgpu_lock)
|->intel_vgpu_page_track_handler (as is kvmgt_gfn_is_write_protected)
   |
   |->page_track->handler() (ppgtt_write_protection_handler())
      |	
      |->ppgtt_handle_guest_write_page_table_bytes()
         |
         |->ppgtt_handle_guest_write_page_table()
	    |
	    |->ppgtt_handle_guest_entry_add() --> new_present
	       |
	       |->ppgtt_populate_spt_by_guest_entry()
	          |
		  |->intel_vgpu_enable_page_track() --> for GFN 2
		     |
		     |->intel_gvt_page_track_add()
		        |
			|->mutex_lock(&info->gfn_lock) ===>deadlock


Below fix based on this patch is to reuse vgpu_lock to protect the hash table
info->ptable.
Please check if it's good.


diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index b924ed079ad4..526bd973e784 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -364,7 +364,7 @@ __kvmgt_protect_table_find(struct intel_vgpu *info, gfn_t gfn)
 {
        struct kvmgt_pgfn *p, *res = NULL;

-       lockdep_assert_held(&info->gfn_lock);
+       lockdep_assert_held(&info->vgpu_lock);

        hash_for_each_possible(info->ptable, p, hnode, gfn) {
                if (gfn == p->gfn) {
@@ -388,7 +388,7 @@ static void kvmgt_protect_table_add(struct intel_vgpu *info, gfn_t gfn)
 {
        struct kvmgt_pgfn *p;

-       lockdep_assert_held(&info->gfn_lock);
+       lockdep_assert_held(&info->vgpu_lock);

        if (kvmgt_gfn_is_write_protected(info, gfn))
                return;
@@ -1572,7 +1572,7 @@ int intel_gvt_page_track_add(struct intel_vgpu *info, u64 gfn)
        if (!info->attached)
                return -ESRCH;

-       mutex_lock(&info->gfn_lock);
+       lockdep_assert_held(&info->vgpu_lock);

        if (kvmgt_gfn_is_write_protected(info, gfn))
                goto out;
@@ -1581,7 +1581,6 @@ int intel_gvt_page_track_add(struct intel_vgpu *info, u64 gfn)
        if (!ret)
                kvmgt_protect_table_add(info, gfn);
 out:
-       mutex_unlock(&info->gfn_lock);
        return ret;
 }

@@ -1592,7 +1591,7 @@ int intel_gvt_page_track_remove(struct intel_vgpu *info, u64 gfn)
        if (!info->attached)
                return 0;

-       mutex_lock(&info->gfn_lock);
+       lockdep_assert_held(&info->vgpu_lock);

        if (!kvmgt_gfn_is_write_protected(info, gfn))
                goto out;
@@ -1601,7 +1600,6 @@ int intel_gvt_page_track_remove(struct intel_vgpu *info, u64 gfn)
        if (!ret)
                kvmgt_protect_table_del(info, gfn);
 out:
-       mutex_unlock(&info->gfn_lock);
        return ret;
 }

@@ -1612,13 +1610,15 @@ static void kvmgt_page_track_write(gpa_t gpa, const u8 *val, int len,
                container_of(node, struct intel_vgpu, track_node);

        mutex_lock(&info->vgpu_lock);
-       mutex_lock(&info->gfn_lock);

        if (kvmgt_gfn_is_write_protected(info, gpa >> PAGE_SHIFT))
                intel_vgpu_page_track_handler(info, gpa,
                                                     (void *)val, len);
        }

-       mutex_unlock(&info->gfn_lock);
        mutex_unlock(&info->vgpu_lock);
 }
@@ -1629,12 +1629,11 @@ static void kvmgt_page_track_remove_region(gfn_t gfn, unsigned long nr_pages,
        struct intel_vgpu *info =
                container_of(node, struct intel_vgpu, track_node);
 
-       mutex_lock(&info->gfn_lock);
+       lockdep_assert_held(&info->vgpu_lock);
        for (i = 0; i < nr_pages; i++) {
                if (kvmgt_gfn_is_write_protected(info, gfn + i))
                        kvmgt_protect_table_del(info, gfn + i);
        }
-       mutex_unlock(&info->gfn_lock);
 }


Thanks
Yan