Re: [Xen-devel] [PATCH] x86/mm: Skip the hypervisor range when walking PGD

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/05/2015 05:31 PM, H. Peter Anvin wrote:
> On 11/05/15 10:56, Boris Ostrovsky wrote:
> > The range between 0xffff800000000000 and 0xffff87ffffffffff is reserved
> > for hypervisor and therefore we should not try to follow PGD's indexes
> > corresponding to those addresses.
> >
> > While this has alsways been a problem, with commit e1a58320a38d ("x86/mm:
> > Warn on W^X mappings") ptdump_walk_pgd_level_core() can now be called
> > during boot, causing a PV Xen guest to crash.
> >
> > Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > ---
> >   arch/x86/mm/dump_pagetables.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
> > index 1bf417e..756c921 100644
> > --- a/arch/x86/mm/dump_pagetables.c
> > +++ b/arch/x86/mm/dump_pagetables.c
> > @@ -362,8 +362,13 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, 
> > pgd_t *pgd,
> >                                        bool checkwx)
> >   {
> >   #ifdef CONFIG_X86_64
> > +/* ffff800000000000 - ffff87ffffffffff is reserved for hypervisor */
> > +#define is_hypervisor_range(idx) (paravirt_enabled() && \
> > +                                 (((idx) >= pgd_index(__PAGE_OFFSET) - 16) && \
> > +                                  ((idx) < pgd_index(__PAGE_OFFSET))))
> >         pgd_t *start = (pgd_t *) &init_level4_pgt;
> >   #else
> > +#define is_hypervisor_range(idx)   0
> >         pgd_t *start = swapper_pg_dir;
> >   #endif
> >         pgprotval_t prot;
> > @@ -381,7 +386,7 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, 
> > pgd_t *pgd,
> >   
> >   	for (i = 0; i < PTRS_PER_PGD; i++) {
> >                 st.current_address = normalize_addr(i * PGD_LEVEL_MULT);
> > -               if (!pgd_none(*start)) {
> > +               if (!pgd_none(*start) && !is_hypervisor_range(i)) {
> >                         if (pgd_large(*start) || !pgd_present(*start)) {
> >                                 prot = pgd_flags(*start);
> >                                 note_page(m, &st, __pgprot(prot), 1);
> Maybe we could use the max_lines field in the address_markers[] array?
> We really shouldn't be mapping anything in the hypervisor space even on
> native.
You mean overload max_lines with a value indicating that the range needs 
to be skipped?
That would require checking the range on each loop iteration since we 
update st.marker *after* we've walked a particular index. (And I think 
it would need to be done on each level to be generic).
I could just drop paravirt_enabled() in is_hypervisor_range() but you 
are thinking about avoiding the macro altogether, right?
(I do need to add hypervisor range to address_markers[])

-boris


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