Re: [Xen-devel] Question Regarding set_gdt

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

It's a bug. I moved the zeroing before get_page_and_type as those
entries could contain garbage in some cases. I forgot to hoist an
appropriate ownership check at the same time. :-)

I'll fix that...

 -- Keir

> It seems to me that set_gdt in xen/arch/x86/x86_32/mm.c could allow a
> domain to partially zero a page that it doesn't own:
> 
>     long set_gdt(struct domain *d, 
>                  unsigned long *frames,
>                  unsigned int entries)
>     {
>         /* NB. There are 512 8-byte entries per GDT page. */
>         int i, nr_pages = (entries + 511) / 512;
>         struct desc_struct *vgdt;
> 
>         vgdt = map_domain_mem(frames[0] << PAGE_SHIFT);
>         memset( vgdt + FIRST_RESERVED_GDT_ENTRY, 0,     
>                NR_RESERVED_GDT_ENTRIES*8);
> 
>         /* Check the new GDT. */
>         for ( i = 0; i < nr_pages; i++ )
>         {
>             if ( unlikely(frames[i] >= max_page) ||
>                  unlikely(!get_page_and_type(&frame_table[frames[i]], 
>                                              d, PGT_gdt_page)) )
>                 goto fail;
>         }
> 
> Xen doesn't seem to check whether the list of pages specified in
> frames[] belongs to the calling domain until get_page_and_type is
> called, but Xen zeroes the reserved GDT entries in the first specified
> page before this.  I don't see any checks in do_set_gdt either.
> 
> Am I missing something, or is this a bug?
> 
> --Michael Vrable
> 
> 
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