[PATCHv2 18/59] x86/mm: Calculate direct mapping size

Kirill A. Shutemov
2019-07-31T15:09:46Z

The kernel needs to have a way to access encrypted memory. We have two
option on how approach it:

- Create temporary mappings every time kernel needs access to encrypted
memory. That's basically brings highmem and its overhead back.

- Create multiple direct mappings, one per-KeyID. In this setup we
don't need to create temporary mappings on the fly -- encrypted
memory is permanently available in kernel address space.

We take the second approach as it has lower overhead.

It's worth noting that with per-KeyID direct mappings compromised kernel
would give access to decrypted data right away without additional tricks
to get memory mapped with the correct KeyID.

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.

A direct mapping for each KeyID will be put next to each other in the
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.

Size of direct mapping is calculated during KASLR setup. If KALSR is
disabled it happens during MKTME initialization.

With MKTME size of direct mapping has to be power-of-2. It makes
implementation of __pa() efficient.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
Documentation/x86/x86_64/mm.rst | 4 +++
arch/x86/include/asm/page_32.h | 1 +
arch/x86/include/asm/page_64.h | 2 ++
arch/x86/include/asm/setup.h | 6 ++++
arch/x86/kernel/head64.c | 4 +++
arch/x86/kernel/setup.c | 3 ++
arch/x86/mm/init_64.c | 58 +++++++++++++++++++++++++++++++++
arch/x86/mm/kaslr.c | 11 +++++--
8 files changed, 86 insertions(+), 3 deletions(-)

diff --git a/Documentation/x86/x86_64/mm.rst b/Documentation/x86/x86_64/mm.rst
index 267fc4808945..7978afe6c396 100644
--- a/Documentation/x86/x86_64/mm.rst
+++ b/Documentation/x86/x86_64/mm.rst
@@ -140,6 +140,10 @@ The direct mapping covers all memory in the system up to the highest
memory address (this means in some cases it can also include PCI memory
holes).

+With MKTME, we have multiple direct mappings. One per-KeyID. They are put
+next to each other. PAGE_OFFSET + N * direct_mapping_size can be used to
+find direct mapping for KeyID-N.
+
vmalloc space is lazily synchronized into the different PML4/PML5 pages of
the processes using the page fault handler, with init_top_pgt as
reference.
diff --git a/arch/x86/include/asm/page_32.h b/arch/x86/include/asm/page_32.h
index 94dbd51df58f..8bce788f9ca9 100644
--- a/arch/x86/include/asm/page_32.h
+++ b/arch/x86/include/asm/page_32.h
@@ -6,6 +6,7 @@

#ifndef __ASSEMBLY__

+#define direct_mapping_size 0
#define __phys_addr_nodebug(x) ((x) - PAGE_OFFSET)
#ifdef CONFIG_DEBUG_VIRTUAL
extern unsigned long __phys_addr(unsigned long);
diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index 939b1cff4a7b..f57fc3cc2246 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -14,6 +14,8 @@ extern unsigned long phys_base;
extern unsigned long page_offset_base;
extern unsigned long vmalloc_base;
extern unsigned long vmemmap_base;
+extern unsigned long direct_mapping_size;
+extern unsigned long direct_mapping_mask;

static inline unsigned long __phys_addr_nodebug(unsigned long x)
{
diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index ed8ec011a9fd..d2861074cf83 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -62,6 +62,12 @@ extern void x86_ce4100_early_setup(void);
static inline void x86_ce4100_early_setup(void) { }
#endif

+#ifdef CONFIG_MEMORY_PHYSICAL_PADDING
+void calculate_direct_mapping_size(void);
+#else
+static inline void calculate_direct_mapping_size(void) { }
+#endif
+
#ifndef _SETUP

#include <asm/espfix.h>
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 29ffa495bd1c..006d3ff46afe 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -60,6 +60,10 @@ EXPORT_SYMBOL(vmalloc_base);
unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
EXPORT_SYMBOL(vmemmap_base);
#endif
+unsigned long direct_mapping_size __ro_after_init = -1UL;
+EXPORT_SYMBOL(direct_mapping_size);
+unsigned long direct_mapping_mask __ro_after_init = -1UL;
+EXPORT_SYMBOL(direct_mapping_mask);

#define __head __section(.head.text)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index bbe35bf879f5..d12431e20876 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1077,6 +1077,9 @@ void __init setup_arch(char **cmdline_p)
*/
init_cache_modes();

+ /* direct_mapping_size has to be initialized before KASLR and MKTME */
+ calculate_direct_mapping_size();
+
/*
* Define random base addresses for memory sections after max_pfn is
* defined and before each memory section base is used.
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index a6b5c653727b..4c1f93df47a5 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1440,6 +1440,64 @@ unsigned long memory_block_size_bytes(void)
return memory_block_size_probed;
}

+#ifdef CONFIG_MEMORY_PHYSICAL_PADDING
+void __init calculate_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 system has? */
+ direct_mapping_size = max_pfn << PAGE_SHIFT;
+ direct_mapping_size = round_up(direct_mapping_size, 1UL << 40);
+
+ if (!mktme_nr_keyids())
+ goto out;
+
+ /*
+ * For MKTME we need direct_mapping_size to be power-of-2.
+ * It makes __pa() implementation efficient.
+ */
+ direct_mapping_size = roundup_pow_of_two(direct_mapping_size);
+
+ /*
+ * 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 (mktme_nr_keyids())
+ direct_mapping_size = roundup_pow_of_two(direct_mapping_size);
+
+ if (direct_mapping_size > available_va)
+ direct_mapping_size = available_va;
+
+ /*
+ * For MKTME, make sure direct_mapping_size is still power-of-2
+ * after adding padding and calculate mask that is used in __pa().
+ */
+ if (mktme_nr_keyids()) {
+ direct_mapping_size = rounddown_pow_of_two(direct_mapping_size);
+ direct_mapping_mask = direct_mapping_size - 1;
+ }
+}
+#endif
+
#ifdef CONFIG_SPARSEMEM_VMEMMAP
/*
* Initialise the sparsemem vmemmap using huge-pages at the PMD level.
diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 580b82c2621b..83af41d289ed 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -103,10 +103,15 @@ 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;

- /* Adapt phyiscal memory region size based on available memory */
+ /*
+ * Calculate space required to map all physical memory.
+ * In case of MKTME, we map physical memory multiple times, one for
+ * each KeyID. If MKTME is disabled mktme_nr_keyids() is 0.
+ */
+ memory_tb = (direct_mapping_size * (mktme_nr_keyids() + 1)) >> TB_SHIFT;
+
+ /* Adapt physical memory region size based on available memory */
if (memory_tb < kaslr_regions[0].size_tb)
kaslr_regions[0].size_tb = memory_tb;

--
2.21.0