RE: [Xen-devel] bug in LDT error checking on stable branch

[Kip Macy <kmacy@xxxxxxxxxxx>]

On Wed, 9 Feb 2005, Ian Pratt wrote:

>
> > do_mmu_update blindly assumes that setting the LDT will
> > always succeed:
>
> It's deferring the operation for performance reasons, which makes it
> tricky to tell the guest whether it failed or not. I guess we could
> return an error for the whole hypercall batch, but at the point the
> failure is detected it would be slighty messy to report which op failed.
>
> I guess the current behaviour is a pain if you're debugging a guest
> port...

Yes. FreeBSD 5.3 added a "nice" optimization (not in 5.2.1) to a free
path where it doesn't unmap the page, leaving a dangling RW mapping.
Perfectly legitimate running directly on hardware, but when the physical
page got reallocated for an LDT running under xen ... So instead we hit
a read fault on loading gs somewhat later - very confusing.



                                        -Kip


>
> Ian
>
> > under MMUEXT_SET_LDT:
> >           if ( ents != 0 )
> >                 percpu_info[cpu].deferred_ops |= DOP_RELOAD_LDT;
> >
> > further down:
> >    if ( deferred_ops & DOP_RELOAD_LDT )
> >         (void)map_ldt_shadow_page(0);
> >
> > if xen thinks that one of the pages passed is mapped r/w somewhere the
> > operation will fail:
> > /* Map shadow page at offset @off. */
> > int map_ldt_shadow_page(unsigned int off)
> > {
> > <...>
> >
> >     if ( unlikely(!(l1e & _PAGE_PRESENT)) ||
> >          unlikely(!get_page_and_type(&frame_table[l1e >> PAGE_SHIFT],
> >                                      d, PGT_ldt_page)) )
> >         return 0;
> >
> > <...>
> >
> > The user won't know about it until he re-loads a segment register or
> > successfully infers what operation the "Bad type" messages on the
> > console refer to.
> >
> >
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