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

[Sean Christopherson <seanjc@google.com>]

On Wed, Dec 28, 2022, Yan Zhao wrote:
> 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

Or even more simply, 

  kvmgt_page_track_write()
  |
  -> intel_vgpu_page_track_handler()
     |
     -> intel_gvt_page_track_remove()

> 
> 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);

I'll just delete these assertions, the one in __kvmgt_protect_table_find() should
cover everything and is ultimately the assert that matters.

> @@ -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);

This path needs to manually take vgpu_lock as it's called from KVM.  IIRC, this
is the main reason I tried adding a new lock.  That and I had a hell of a time
figuring out whether or not vgpu_lock would actually be held.

Looking at this with fresh eyes, AFAICT intel_vgpu_reset_gtt() is the only other
path that can reach __kvmgt_protect_table_find() without holding vgpu_lock, by
way of intel_gvt_page_track_remove().  But unless there's magic I'm missing, that's
dead code and can simply be deleted.