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

Yan Zhao
2022-12-28T05:26:46Z

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