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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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...

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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