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Subject[PATCH 03/11] x86/mm: introduce "default" kernel PTE mask
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From: Dave Hansen <dave.hansen@linux.intel.com>

The __PAGE_KERNEL_* page permissions are "raw". They contain bits
that may or may not be supported on the current processor. They need
to be filtered by a mask (currently __supported_pte_mask) to turn them
into a value that we can actually set in a PTE.

These __PAGE_KERNEL_* values all contain _PAGE_GLOBAL. But, with PTI,
we want to be able to support _PAGE_GLOBAL (have the bit set in
__supported_pte_mask) but not have it appear in any of these masks by
default.

This patch creates a new mask, __default_kernel_pte_mask, and applies
it when creating all of the PAGE_KERNEL_* masks. This makes
PAGE_KERNEL_* safe to use anywhere (they only contain supported bits).
It also ensures that PAGE_KERNEL_* contains _PAGE_GLOBAL on PTI=n
kernels but clears _PAGE_GLOBAL when PTI=y.

We also make __default_kernel_pte_mask a non-GPL exported symbol
because there are plenty of driver-available interfaces that take
PAGE_KERNEL_* permissions.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.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
Cc: Nadav Amit <namit@vmware.com>
---

b/arch/x86/include/asm/pgtable_types.h | 27 +++++++++++++++------------
b/arch/x86/mm/init.c | 6 ++++++
b/arch/x86/mm/init_32.c | 8 +++++++-
b/arch/x86/mm/init_64.c | 5 +++++
4 files changed, 33 insertions(+), 13 deletions(-)

diff -puN arch/x86/include/asm/pgtable_types.h~KERN-pgprot-default arch/x86/include/asm/pgtable_types.h
--- a/arch/x86/include/asm/pgtable_types.h~KERN-pgprot-default 2018-04-02 16:41:13.662605176 -0700
+++ b/arch/x86/include/asm/pgtable_types.h 2018-04-02 16:41:13.672605176 -0700
@@ -196,19 +196,21 @@ enum page_cache_mode {
#define __PAGE_KERNEL_NOENC (__PAGE_KERNEL)
#define __PAGE_KERNEL_NOENC_WP (__PAGE_KERNEL_WP)

-#define PAGE_KERNEL __pgprot(__PAGE_KERNEL | _PAGE_ENC)
-#define PAGE_KERNEL_NOENC __pgprot(__PAGE_KERNEL)
-#define PAGE_KERNEL_RO __pgprot(__PAGE_KERNEL_RO | _PAGE_ENC)
-#define PAGE_KERNEL_EXEC __pgprot(__PAGE_KERNEL_EXEC | _PAGE_ENC)
-#define PAGE_KERNEL_EXEC_NOENC __pgprot(__PAGE_KERNEL_EXEC)
-#define PAGE_KERNEL_RX __pgprot(__PAGE_KERNEL_RX | _PAGE_ENC)
-#define PAGE_KERNEL_NOCACHE __pgprot(__PAGE_KERNEL_NOCACHE | _PAGE_ENC)
-#define PAGE_KERNEL_LARGE __pgprot(__PAGE_KERNEL_LARGE | _PAGE_ENC)
-#define PAGE_KERNEL_LARGE_EXEC __pgprot(__PAGE_KERNEL_LARGE_EXEC | _PAGE_ENC)
-#define PAGE_KERNEL_VVAR __pgprot(__PAGE_KERNEL_VVAR | _PAGE_ENC)
+#define default_pgprot(x) __pgprot((x) & __default_kernel_pte_mask)

-#define PAGE_KERNEL_IO __pgprot(__PAGE_KERNEL_IO)
-#define PAGE_KERNEL_IO_NOCACHE __pgprot(__PAGE_KERNEL_IO_NOCACHE)
+#define PAGE_KERNEL default_pgprot(__PAGE_KERNEL | _PAGE_ENC)
+#define PAGE_KERNEL_NOENC default_pgprot(__PAGE_KERNEL)
+#define PAGE_KERNEL_RO default_pgprot(__PAGE_KERNEL_RO | _PAGE_ENC)
+#define PAGE_KERNEL_EXEC default_pgprot(__PAGE_KERNEL_EXEC | _PAGE_ENC)
+#define PAGE_KERNEL_EXEC_NOENC default_pgprot(__PAGE_KERNEL_EXEC)
+#define PAGE_KERNEL_RX default_pgprot(__PAGE_KERNEL_RX | _PAGE_ENC)
+#define PAGE_KERNEL_NOCACHE default_pgprot(__PAGE_KERNEL_NOCACHE | _PAGE_ENC)
+#define PAGE_KERNEL_LARGE default_pgprot(__PAGE_KERNEL_LARGE | _PAGE_ENC)
+#define PAGE_KERNEL_LARGE_EXEC default_pgprot(__PAGE_KERNEL_LARGE_EXEC | _PAGE_ENC)
+#define PAGE_KERNEL_VVAR default_pgprot(__PAGE_KERNEL_VVAR | _PAGE_ENC)
+
+#define PAGE_KERNEL_IO default_pgprot(__PAGE_KERNEL_IO)
+#define PAGE_KERNEL_IO_NOCACHE default_pgprot(__PAGE_KERNEL_IO_NOCACHE)

#endif /* __ASSEMBLY__ */

@@ -483,6 +485,7 @@ static inline pgprot_t pgprot_large_2_4k
typedef struct page *pgtable_t;

extern pteval_t __supported_pte_mask;
+extern pteval_t __default_kernel_pte_mask;
extern void set_nx(void);
extern int nx_enabled;

diff -puN arch/x86/mm/init_32.c~KERN-pgprot-default arch/x86/mm/init_32.c
--- a/arch/x86/mm/init_32.c~KERN-pgprot-default 2018-04-02 16:41:13.664605176 -0700
+++ b/arch/x86/mm/init_32.c 2018-04-02 16:41:13.672605176 -0700
@@ -558,8 +558,14 @@ static void __init pagetable_init(void)
permanent_kmaps_init(pgd_base);
}

-pteval_t __supported_pte_mask __read_mostly = ~(_PAGE_NX | _PAGE_GLOBAL);
+#define DEFAULT_PTE_MASK ~(_PAGE_NX | _PAGE_GLOBAL)
+/* Bits supported by the hardware: */
+pteval_t __supported_pte_mask __read_mostly = DEFAULT_PTE_MASK;
+/* Bits allowed in normal kernel mappings: */
+pteval_t __default_kernel_pte_mask __read_mostly = DEFAULT_PTE_MASK;
EXPORT_SYMBOL_GPL(__supported_pte_mask);
+/* Used in PAGE_KERNEL_* macros which are reasonably used out-of-tree: */
+EXPORT_SYMBOL(__default_kernel_pte_mask);

/* user-defined highmem size */
static unsigned int highmem_pages = -1;
diff -puN arch/x86/mm/init_64.c~KERN-pgprot-default arch/x86/mm/init_64.c
--- a/arch/x86/mm/init_64.c~KERN-pgprot-default 2018-04-02 16:41:13.666605176 -0700
+++ b/arch/x86/mm/init_64.c 2018-04-02 16:41:13.673605176 -0700
@@ -65,8 +65,13 @@
* around without checking the pgd every time.
*/

+/* Bits supported by the hardware: */
pteval_t __supported_pte_mask __read_mostly = ~0;
+/* Bits allowed in normal kernel mappings: */
+pteval_t __default_kernel_pte_mask __read_mostly = ~0;
EXPORT_SYMBOL_GPL(__supported_pte_mask);
+/* Used in PAGE_KERNEL_* macros which are reasonably used out-of-tree: */
+EXPORT_SYMBOL(__default_kernel_pte_mask);

int force_personality32;

diff -puN arch/x86/mm/init.c~KERN-pgprot-default arch/x86/mm/init.c
--- a/arch/x86/mm/init.c~KERN-pgprot-default 2018-04-02 16:41:13.668605176 -0700
+++ b/arch/x86/mm/init.c 2018-04-02 16:41:13.673605176 -0700
@@ -190,6 +190,12 @@ static void __init probe_page_size_mask(
enable_global_pages();
}

+ /* By the default is everything supported: */
+ __default_kernel_pte_mask = __supported_pte_mask;
+ /* Except when with PTI where the kernel is mostly non-Global: */
+ if (cpu_feature_enabled(X86_FEATURE_PTI))
+ __default_kernel_pte_mask &= ~_PAGE_GLOBAL;
+
/* Enable 1 GB linear kernel mappings if available: */
if (direct_gbpages && boot_cpu_has(X86_FEATURE_GBPAGES)) {
printk(KERN_INFO "Using GB pages for direct mapping\n");
_
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