Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory

[Chao Peng]

On Tue, Jan 17, 2023 at 04:34:15PM +0000, Sean Christopherson wrote:
> On Tue, Jan 17, 2023, Chao Peng wrote:
> > On Fri, Jan 13, 2023 at 09:54:41PM +0000, Sean Christopherson wrote:
> > > > +       list_for_each_entry(notifier, &data->notifiers, list) {
> > > > +               notifier->ops->invalidate_start(notifier, start, end);
> > > 
> > > Two major design issues that we overlooked long ago:
> > > 
> > >   1. Blindly invoking notifiers will not scale.  E.g. if userspace 
> > > configures a
> > >      VM with a large number of convertible memslots that are all backed 
> > > by a
> > >      single large restrictedmem instance, then converting a single page 
> > > will
> > >      result in a linear walk through all memslots.  I don't expect anyone 
> > > to
> > >      actually do something silly like that, but I also never expected 
> > > there to be
> > >      a legitimate usecase for thousands of memslots.
> > > 
> > >   2. This approach fails to provide the ability for KVM to ensure a guest 
> > > has
> > >      exclusive access to a page.  As discussed in the past, the kernel 
> > > can rely
> > >      on hardware (and maybe ARM's pKVM implementation?) for those 
> > > guarantees, but
> > >      only for SNP and TDX VMs.  For VMs where userspace is trusted to 
> > > some extent,
> > >      e.g. SEV, there is value in ensuring a 1:1 association.
> > > 
> > >      And probably more importantly, relying on hardware for SNP and TDX 
> > > yields a
> > >      poor ABI and complicates KVM's internals.  If the kernel doesn't 
> > > guarantee a
> > >      page is exclusive to a guest, i.e. if userspace can hand out the 
> > > same page
> > >      from a restrictedmem instance to multiple VMs, then failure will 
> > > occur only
> > >      when KVM tries to assign the page to the second VM.  That will 
> > > happen deep
> > >      in KVM, which means KVM needs to gracefully handle such errors, and 
> > > it means
> > >      that KVM's ABI effectively allows plumbing garbage into its memslots.
> > 
> > It may not be a valid usage, but in my TDX environment I do meet below
> > issue.
> > 
> > kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x4 gpa=0x0 size=0x80000000 
> > ua=0x7fe1ebfff000 ret=0
> > kvm_set_user_memory AddrSpace#0 Slot#1 flags=0x4 gpa=0xffc00000 
> > size=0x400000 ua=0x7fe271579000 ret=0
> > kvm_set_user_memory AddrSpace#0 Slot#2 flags=0x4 gpa=0xfeda0000 
> > size=0x20000 ua=0x7fe1ec09f000 ret=-22
> > 
> > Slot#2('SMRAM') is actually an alias into system memory(Slot#0) in QEMU
> > and slot#2 fails due to below exclusive check.
> > 
> > Currently I changed QEMU code to mark these alias slots as shared
> > instead of private but I'm not 100% confident this is correct fix.
> 
> That's a QEMU bug of sorts.  SMM is mutually exclusive with TDX, QEMU 
> shouldn't
> be configuring SMRAM (or any SMM memslots for that matter) for TDX guests.

Thanks for the confirmation. As long as we only bind one notifier for
each address, using xarray does make things simple.

Chao