Re: [Xen-devel] [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/21/2015 08:49 PM, Andrew Cooper wrote:
> On 22/07/2015 01:28, Andy Lutomirski wrote:
> > On Tue, Jul 21, 2015 at 5:21 PM, Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx> wrote:
> > > On 22/07/2015 01:07, Andy Lutomirski wrote:
> > > > On Tue, Jul 21, 2015 at 4:38 PM, Andrew Cooper
> > > > <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > On 21/07/2015 22:53, Boris Ostrovsky wrote:
> > > > > > On 07/21/2015 03:59 PM, Andy Lutomirski wrote:
> > > > > > > --- a/arch/x86/include/asm/mmu_context.h
> > > > > > > +++ b/arch/x86/include/asm/mmu_context.h
> > > > > > > @@ -34,6 +34,44 @@ static inline void load_mm_cr4(struct mm_struct
> > > > > > > *mm) {}
> > > > > > >    #endif
> > > > > > >      /*
> > > > > > > + * ldt_structs can be allocated, used, and freed, but they are never
> > > > > > > + * modified while live.
> > > > > > > + */
> > > > > > > +struct ldt_struct {
> > > > > > > +    int size;
> > > > > > > +    int __pad;    /* keep the descriptors naturally aligned. */
> > > > > > > +    struct desc_struct entries[];
> > > > > > > +};
> > > > > > This breaks Xen which expects LDT to be page-aligned. Not sure why.
> > > > > >
> > > > > > Jan, Andrew?
> > > > > PV guests are not permitted to have writeable mappings to the frames
> > > > > making up the GDT and LDT, so it cannot make unaudited changes to
> > > > > loadable descriptors.  In particular, for a 32bit PV guest, it is only
> > > > > the segment limit which protects Xen from the ring1 guest kernel.
> > > > >
> > > > > A lot of this code hasn't been touched in years, and it certainly
> > > > > predates me.  The alignment requirement appears to come from the virtual
> > > > > region Xen uses to map the guests GDT and LDT.  Strict alignment is
> > > > > required for the GDT so Xen's descriptors starting at 0xe0xx are
> > > > > correct, but the LDT alignment seems to be a side effect of similar
> > > > > codepaths.
> > > > >
> > > > > For an LDT smaller than 8192 entries, I can't see any specific reason
> > > > > for enforcing alignment, other than "that's the way it has always been".
> > > > >
> > > > > However, the guest would still have to relinquish write access to all
> > > > > frames which make up the LDT, which looks to be a bit of an issue given
> > > > > the snippet above.
> > > > Does the LDT itself need to be aligned or just the address passed to
> > > > paravirt_alloc_ldt?
> > > The address which Xen receives needs to be aligned.
> > >
> > > It looks like xen_alloc_ldt() blindly assumes that the desc_struct *ldt
> > > it is passed is page aligned, and passes it straight through.
> > xen_alloc_ldt is just fiddling with protection though, I think.  Isn't
> > it xen_set_ldt that's the meat?  We could easily pass xen_alloc_ldt a
> > pointer to the ldt_struct.
> So it is.  It is the linear_addr in xen_set_ldt() which Xen currently
> audits to be page aligned.
>
> > > > > This will allow ldt_struct itself to be page aligned, and for the size
> > > > > field to sit across the base/limit field of what would logically be
> > > > > selector 0x0008  There would be some issues accessing size.  To load
> > > > > frames as an LDT, a guest must drop all refs to the page so that its
> > > > > type may be changed from writeable to segdesc.  After that, an
> > > > > update_descriptor hypercall can be used to change size, and I believe
> > > > > the guest may subsequently recreate read-only mappings to the frames in
> > > > > question (although frankly it is getting late so you will want to double
> > > > > check all of this).
> > > > >
> > > > > Anyhow, this looks like an issue which should be fixed up with slightly
> > > > > more PVOps, rather than enforcing a Xen view of the world on native Linux.
> > > > I could presumably make the allocation the other way around so the
> > > > size is at the end.  I could even use two separate allocations if
> > > > needed.
Why not wrap mm_context_t's ldt and size into a struct (just like 
ldt_struct but without __pad) and have a single allocation of ldt?
I.e.

struct ldt_struct {
    int size;
    struct desc_struct *entries;
}

--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -9,8 +9,7 @@
   * we put the segment information here.
   */
  typedef struct {
-    void *ldt;
-    int size;
+    struct ldt_struct ldt;
    #ifdef CONFIG_X86_64
      /* True if mm supports a task running in 32 bit compatibility 
mode. */

-boris

> > > I suspect two separate allocations would be the better solution, as it
> > > means that the size field doesn't need to be subject to funny page
> > > permissions.
> > True.  OTOH we never write to the size field after allocating the thing.
> Right, but even reading it is going to cause problems if one of the
> paravirt ops can't re-establish ro mappings.
>
> ~Andrew
>
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