Re: [PATCH v3 09/21] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]

[Sean Christopherson]

On Fri, Jan 31, 2020 at 03:55:50PM -0500, Peter Xu wrote:
> On Fri, Jan 31, 2020 at 12:36:22PM -0800, Sean Christopherson wrote:
> > On Fri, Jan 31, 2020 at 03:28:24PM -0500, Peter Xu wrote:
> > > On Fri, Jan 31, 2020 at 11:33:01AM -0800, Sean Christopherson wrote:
> > > > For the same reason we don't take mmap_sem, it gains us nothing, i.e. KVM
> > > > still has to use copy_{to,from}_user().
> > > > 
> > > > In the proposed __x86_set_memory_region() refactor, vmx_set_tss_addr()
> > > > would be provided the hva of the memory region.  Since slots_lock and SRCU
> > > > only protect gfn->hva, why would KVM take slots_lock since it already has
> > > > the hva?
> > > 
> > > OK so you're suggesting to unlock the lock earlier to not cover
> > > init_rmode_tss() rather than dropping the whole lock...  Yes it looks
> > > good to me.  I think that's the major confusion I got.
> > 
> > Ya.  And I missed where the -EEXIST was coming from.  I think we're on the
> > same page.
> 
> Good to know.  Btw, for me I would still prefer to keep the lock be
> after the __copy_to_user()s because "HVA is valid without lock" is
> only true for these private memslots.

No.  From KVM's perspective, the HVA is *never* valid.  Even if you rewrote
this statement to say "the gfn->hva translation is valid without lock" it
would still be incorrect. 

KVM is *always* using HVAs without holding lock, e.g. every time it enters
the guest it is deferencing a memslot because the translations stored in
the TLB are effectively gfn->hva->hpa.  Obviously KVM ensures that it won't
dereference a memslot that has been deleted/moved, but it's a lot more
subtle than simply holding a lock.

> After all this is super slow path so I wouldn't mind to take the lock
> for some time longer.

Holding the lock doesn't affect this super slow vmx_set_tss_addr(), it
affects everything else that wants slots_lock.  Now, admittedly it's
extremely unlikely userspace is going to do KVM_SET_USER_MEMORY_REGION in
parallel, but that's not the point and it's not why I'm objecting to
holding the lock.

Holding the lock implies protection that is *not* provided.  You and I know
it's not needed for copy_{to,from}_user(), but look how long it's taken us
to get on the same page.  A future KVM developer comes along, sees this
code, and thinks "oh, I need to hold slots_lock to dereference a gfn", and
propagates the unnecessary locking to some other code.

> Or otherwise if you really like the unlock() to
> be earlier I can comment above the unlock:
> 
>   /*
>    * We can unlock before using the HVA only because this KVM private
>    * memory slot will never change until the end of VM lifecycle.
>    */

How about:

	/*
	 * No need to hold slots_lock while filling the TSS, the TSS private
	 * memslot is guaranteed to be valid until the VM is destroyed, i.e.
	 * there is no danger of corrupting guest memory by consuming a stale
	 * gfn->hva lookup.
	 */