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Subject[PATCH 2/3] x86/mm: introduce __PAGE_KERNEL_GLOBAL
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From: Dave Hansen <dave.hansen@linux.intel.com>

Kernel mappings are historically _PAGE_GLOBAL. But, with PTI, we do not
want them to be _PAGE_GLOBAL. We currently accomplish this by simply
clearing _PAGE_GLOBAL from the suppotred mask which ensures it is
cleansed from many of our PTE construction sites:

if (!static_cpu_has(X86_FEATURE_PTI))
__supported_pte_mask |= _PAGE_GLOBAL;

But, this also means that we now get *no* opportunity to use global
pages with PTI, even for data which is shared such as the cpu_entry_area
and entry/exit text.

This patch introduces a new mask: __PAGE_KERNEL_GLOBAL. This mask
can be thought of as the default global bit value when creating kernel
mappings. We make it _PAGE_GLOBAL when PTI=n, but 0 when PTI=y. This
ensures that on PTI kernels, all of the __PAGE_KERNEL_* users will not
get _PAGE_GLOBAL.

This also restores _PAGE_GLOBAL to __supported_pte_mask, allowing it
to be set in the first place.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: x86@kernel.org
---

b/arch/x86/include/asm/pgtable_types.h | 9 ++++++++-
b/arch/x86/mm/init.c | 8 +-------
b/arch/x86/mm/pageattr.c | 9 +++++----
3 files changed, 14 insertions(+), 12 deletions(-)

diff -puN arch/x86/include/asm/pgtable_types.h~kpti-no-global-for-kernel-mappings arch/x86/include/asm/pgtable_types.h
--- a/arch/x86/include/asm/pgtable_types.h~kpti-no-global-for-kernel-mappings 2018-02-13 15:17:56.144210060 -0800
+++ b/arch/x86/include/asm/pgtable_types.h 2018-02-13 15:17:56.152210060 -0800
@@ -180,8 +180,15 @@ enum page_cache_mode {
#define PAGE_READONLY_EXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | \
_PAGE_ACCESSED)

+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+#define __PAGE_KERNEL_GLOBAL 0
+#else
+#define __PAGE_KERNEL_GLOBAL _PAGE_GLOBAL
+#endif
+
#define __PAGE_KERNEL_EXEC \
- (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_GLOBAL)
+ (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | \
+ __PAGE_KERNEL_GLOBAL)
#define __PAGE_KERNEL (__PAGE_KERNEL_EXEC | _PAGE_NX)

#define __PAGE_KERNEL_RO (__PAGE_KERNEL & ~_PAGE_RW)
diff -puN arch/x86/mm/init.c~kpti-no-global-for-kernel-mappings arch/x86/mm/init.c
--- a/arch/x86/mm/init.c~kpti-no-global-for-kernel-mappings 2018-02-13 15:17:56.146210060 -0800
+++ b/arch/x86/mm/init.c 2018-02-13 15:17:56.152210060 -0800
@@ -162,12 +162,6 @@ struct map_range {

static int page_size_mask;

-static void enable_global_pages(void)
-{
- if (!static_cpu_has(X86_FEATURE_PTI))
- __supported_pte_mask |= _PAGE_GLOBAL;
-}
-
static void __init probe_page_size_mask(void)
{
/*
@@ -189,7 +183,7 @@ static void __init probe_page_size_mask(
__supported_pte_mask &= ~_PAGE_GLOBAL;
if (boot_cpu_has(X86_FEATURE_PGE)) {
cr4_set_bits_and_update_boot(X86_CR4_PGE);
- enable_global_pages();
+ __supported_pte_mask |= _PAGE_GLOBAL;
}

/* Enable 1 GB linear kernel mappings if available: */
diff -puN arch/x86/mm/pageattr.c~kpti-no-global-for-kernel-mappings arch/x86/mm/pageattr.c
--- a/arch/x86/mm/pageattr.c~kpti-no-global-for-kernel-mappings 2018-02-13 15:17:56.148210060 -0800
+++ b/arch/x86/mm/pageattr.c 2018-02-13 15:17:56.153210060 -0800
@@ -593,7 +593,8 @@ try_preserve_large_page(pte_t *kpte, uns
* different bit positions in the two formats.
*/
req_prot = pgprot_4k_2_large(req_prot);
- req_prot = pgprot_set_on_present(req_prot, _PAGE_GLOBAL | _PAGE_PSE);
+ req_prot = pgprot_set_on_present(req_prot,
+ __PAGE_KERNEL_GLOBAL | _PAGE_PSE);
req_prot = canon_pgprot(req_prot);

/*
@@ -703,7 +704,7 @@ __split_large_page(struct cpa_data *cpa,
return 1;
}

- ref_prot = pgprot_set_on_present(ref_prot, _PAGE_GLOBAL);
+ ref_prot = pgprot_set_on_present(ref_prot, __PAGE_KERNEL_GLOBAL);

/*
* Get the target pfn from the original entry:
@@ -926,7 +927,7 @@ static void populate_pte(struct cpa_data

pte = pte_offset_kernel(pmd, start);

- pgprot = pgprot_set_on_present(pgprot, _PAGE_GLOBAL);
+ pgprot = pgprot_set_on_present(pgprot, __PAGE_KERNEL_GLOBAL);
pgprot = canon_pgprot(pgprot);

while (num_pages-- && start < end) {
@@ -1219,7 +1220,7 @@ repeat:

new_prot = static_protections(new_prot, address, pfn);

- new_prot = pgprot_set_on_present(new_prot, _PAGE_GLOBAL);
+ new_prot = pgprot_set_on_present(new_prot, __PAGE_KERNEL_GLOBAL);

/*
* We need to keep the pfn from the existing PTE,
_
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