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

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Date:	Thu, 5 Nov 2015 22:38:20 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de,
	mingo@...hat.com
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, linux@...elenboom.it,
	xen-devel@...ts.xen.org
Subject: Re: [PATCH] x86/mm: Skip the hypervisor range when walking PGD



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@...elenboom.it>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.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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