Re: [PATCH v5 0/9] Enable Sub-page Write Protection Support

[Yang Weijiang]

On Fri, Oct 11, 2019 at 09:11:54AM -0700, Jim Mattson wrote:
> On Fri, Oct 11, 2019 at 12:48 AM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
> >
> > On Thu, Oct 10, 2019 at 02:42:51PM -0700, Jim Mattson wrote:
> > > On Tue, Sep 17, 2019 at 1:52 AM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
> > > >
> > > > EPT-Based Sub-Page write Protection(SPP)is a HW capability which allows
> > > > Virtual Machine Monitor(VMM) to specify write-permission for guest
> > > > physical memory at a sub-page(128 byte) granularity. When this
> > > > capability is enabled, the CPU enforces write-access check for sub-pages
> > > > within a 4KB page.
> > > >
> > > > The feature is targeted to provide fine-grained memory protection for
> > > > usages such as device virtualization, memory check-point and VM
> > > > introspection etc.
> > > >
> > > > SPP is active when the "sub-page write protection" (bit 23) is 1 in
> > > > Secondary VM-Execution Controls. The feature is backed with a Sub-Page
> > > > Permission Table(SPPT), SPPT is referenced via a 64-bit control field
> > > > called Sub-Page Permission Table Pointer (SPPTP) which contains a
> > > > 4K-aligned physical address.
> > > >
> > > > To enable SPP for certain physical page, the gfn should be first mapped
> > > > to a 4KB entry, then set bit 61 of the corresponding EPT leaf entry.
> > > > While HW walks EPT, if bit 61 is set, it traverses SPPT with the guset
> > > > physical address to find out the sub-page permissions at the leaf entry.
> > > > If the corresponding bit is set, write to sub-page is permitted,
> > > > otherwise, SPP induced EPT violation is generated.
> > >
> > > How do you handle sub-page permissions for instructions emulated by kvm?
> > How about checking if the gpa is SPP protected, if it is, inject some
> > exception to guest?
> The SPP semantics are well-defined. If a kvm-emulated instruction
> tries to write to a sub-page that is write-protected, then an
> SPP-induced EPT violation should be synthesized.
Hi, Jim,

Regarding the emulated instructions in KVM, there're quite a few
instructions can write guest memory, such as MOVS, XCHG, INS etc.,
check each destination against SPP protected area would be trivial if
deals with them individually, and PIO/MMIO induced vmexit/page_fault also
can link to a SPP protected page, e.g., a string instruction's the destination
is SPP protected memory. Is there a good way to intercept these writes?
emulate_ops.write_emulated() is called in most of the emulation cases to
check and write guest memory, but not sure it's suitable.
Do you have any suggestion?

Thanks!