Re: [PATCHv3 14/17] x86/mm: Introduce direct_mapping_size

Kirill A. Shutemov
2018-06-18T13:12:54Z

On Wed, Jun 13, 2018 at 06:37:07PM +0000, Dave Hansen wrote:
> On 06/12/2018 07:39 AM, Kirill A. Shutemov wrote:
> > Kernel need to have a way to access encrypted memory. We are going to
> "The kernel needs"...
>
> > use per-KeyID direct mapping to facilitate the access with minimal
> > overhead.
>
> What are the security implications of this approach?

I'll add this to the message:

Per-KeyID mappings require a lot more virtual address space. On 4-level
machine with 64 KeyIDs we max out 46-bit virtual address space dedicated
for direct mapping with 1TiB of RAM. Given that we round up any
calculation on direct mapping size to 1TiB, we effectively claim all
46-bit address space for direct mapping on such machine regardless of
RAM size.

Increased usage of virtual address space has implications for KASLR:
we have less space for randomization. With 64 TiB claimed for direct
mapping with 4-level we left with 27 TiB of entropy to place
page_offset_base, vmalloc_base and vmemmap_base.

5-level paging provides much wider virtual address space and KASLR
doesn't suffer significantly from per-KeyID direct mappings.

It's preferred to run MKTME with 5-level paging.

> > Direct mapping for each KeyID will be put next to each other in the
>
> That needs to be "a direct mapping" or "the direct mapping". It's
> missing an article to start the sentence.

Okay.

> > virtual address space. We need to have a way to find boundaries of
> > direct mapping for particular KeyID.
> >
> > The new variable direct_mapping_size specifies the size of direct
> > mapping. With the value, it's trivial to find direct mapping for
> > KeyID-N: PAGE_OFFSET + N * direct_mapping_size.
>
> I think this deserves an update to Documentation/x86/x86_64/mm.txt, no?

Right, I'll update it.

> > Size of direct mapping is calculated during KASLR setup. If KALSR is
> > disable it happens during MKTME initialization.
>
> "disabled"
>
> > diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> > index 4408cd9a3bef..3d8ef8cb97e1 100644
> > --- a/arch/x86/mm/kaslr.c
> > +++ b/arch/x86/mm/kaslr.c
> > @@ -69,6 +69,15 @@ static inline bool kaslr_memory_enabled(void)
> > return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
> > }
> >
> > +#ifndef CONFIG_X86_INTEL_MKTME
> > +static void __init setup_direct_mapping_size(void)
> > +{
> > + direct_mapping_size = max_pfn << PAGE_SHIFT;
> > + direct_mapping_size = round_up(direct_mapping_size, 1UL << TB_SHIFT);
> > + direct_mapping_size += (1UL << TB_SHIFT) * CONFIG_MEMORY_PHYSICAL_PADDING;
> > +}
> > +#endif
>
> Comments, please.

Okay.

> > /* Initialize base and padding for each memory region randomized with KASLR */
> > void __init kernel_randomize_memory(void)
> > {
> > @@ -93,7 +102,11 @@ void __init kernel_randomize_memory(void)
> > if (!kaslr_memory_enabled())
> > return;
> >
> > - kaslr_regions[0].size_tb = 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT);
> > + /*
> > + * Upper limit for direct mapping size is 1/4 of whole virtual
> > + * address space
> > + */
> > + kaslr_regions[0].size_tb = 1 << (__VIRTUAL_MASK_SHIFT - 1 - TB_SHIFT);
>
> Is this a cleanup that can be separate?

Right. I'll split it up.

> > kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
> >
> > /*
> > @@ -101,8 +114,10 @@ void __init kernel_randomize_memory(void)
> > * add padding if needed (especially for memory hotplug support).
> > */
> > BUG_ON(kaslr_regions[0].base != &page_offset_base);
> > - memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
> > - CONFIG_MEMORY_PHYSICAL_PADDING;
> > +
> > + setup_direct_mapping_size();
> > +
> > + memory_tb = direct_mapping_size * mktme_nr_keyids + 1;
>
> What's the +1 for? Is "mktme_nr_keyids" 0 for "MKTME unsupported"?
> That needs to be called out, I think.

I'll add a comment.

> > /* Adapt phyiscal memory region size based on available memory */
> > if (memory_tb < kaslr_regions[0].size_tb)
> > diff --git a/arch/x86/mm/mktme.c b/arch/x86/mm/mktme.c
> > index 43a44f0f2a2d..3e5322bf035e 100644
> > --- a/arch/x86/mm/mktme.c
> > +++ b/arch/x86/mm/mktme.c
> > @@ -89,3 +89,51 @@ static bool need_page_mktme(void)
> > struct page_ext_operations page_mktme_ops = {
> > .need = need_page_mktme,
> > };
> > +
> > +void __init setup_direct_mapping_size(void)
> > +{
> > + unsigned long available_va;
> > +
> > + /* 1/4 of virtual address space is didicated for direct mapping */
> > + available_va = 1UL << (__VIRTUAL_MASK_SHIFT - 1);
> > +
> > + /* How much memory the systrem has? */
> > + direct_mapping_size = max_pfn << PAGE_SHIFT;
> > + direct_mapping_size = round_up(direct_mapping_size, 1UL << 40);
> > +
> > + if (mktme_status != MKTME_ENUMERATED)
> > + goto out;
> > +
> > + /*
> > + * Not enough virtual address space to address all physical memory with
> > + * MKTME enabled. Even without padding.
> > + *
> > + * Disable MKTME instead.
> > + */
> > + if (direct_mapping_size > available_va / mktme_nr_keyids + 1) {
> > + pr_err("x86/mktme: Disabled. Not enough virtual address space\n");
> > + pr_err("x86/mktme: Consider switching to 5-level paging\n");
> > + mktme_disable();
> > + goto out;
> > + }
> > +
> > + /*
> > + * Virtual address space is divided between per-KeyID direct mappings.
> > + */
> > + available_va /= mktme_nr_keyids + 1;
> > +out:
> > + /* Add padding, if there's enough virtual address space */
> > + direct_mapping_size += (1UL << 40) * CONFIG_MEMORY_PHYSICAL_PADDING;
> > + if (direct_mapping_size > available_va)
> > + direct_mapping_size = available_va;
> > +}
>
> Do you really need two copies of this function? Shouldn't it see
> mktme_status!=MKTME_ENUMERATED and just jump out? How is the code
> before that "goto out" different from the CONFIG_MKTME=n case?

mktme.c is not compiled for CONFIG_MKTME=n.

--
Kirill A. Shutemov