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SubjectRe: [PATCH] x86/mm: Skip the hypervisor range when walking PGD
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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@eikelenboom.it>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> ---
>> 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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