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Subject[patch 31/60] x86/mm/kpti: Add mapping helper functions
From: Dave Hansen <dave.hansen@linux.intel.com>

Add the pagetable helper functions do manage the separate user space page
tables.

[ tglx: Split out from the big combo kaiser patch ]

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
arch/x86/include/asm/pgtable_64.h | 139 ++++++++++++++++++++++++++++++++++++++
1 file changed, 139 insertions(+)

--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -131,9 +131,144 @@ static inline pud_t native_pudp_get_and_
#endif
}

+#ifdef CONFIG_KERNEL_PAGE_TABLE_ISOLATION
+/*
+ * All top-level KERNEL_PAGE_TABLE_ISOLATION page tables are order-1 pages
+ * (8k-aligned and 8k in size). The kernel one is at the beginning 4k and
+ * the user one is in the last 4k. To switch between them, you
+ * just need to flip the 12th bit in their addresses.
+ */
+#define KPTI_PGTABLE_SWITCH_BIT PAGE_SHIFT
+
+/*
+ * This generates better code than the inline assembly in
+ * __set_bit().
+ */
+static inline void *ptr_set_bit(void *ptr, int bit)
+{
+ unsigned long __ptr = (unsigned long)ptr;
+
+ __ptr |= BIT(bit);
+ return (void *)__ptr;
+}
+static inline void *ptr_clear_bit(void *ptr, int bit)
+{
+ unsigned long __ptr = (unsigned long)ptr;
+
+ __ptr &= ~BIT(bit);
+ return (void *)__ptr;
+}
+
+static inline pgd_t *kernel_to_user_pgdp(pgd_t *pgdp)
+{
+ return ptr_set_bit(pgdp, KPTI_PGTABLE_SWITCH_BIT);
+}
+
+static inline pgd_t *user_to_kernel_pgdp(pgd_t *pgdp)
+{
+ return ptr_clear_bit(pgdp, KPTI_PGTABLE_SWITCH_BIT);
+}
+
+static inline p4d_t *kernel_to_user_p4dp(p4d_t *p4dp)
+{
+ return ptr_set_bit(p4dp, KPTI_PGTABLE_SWITCH_BIT);
+}
+
+static inline p4d_t *user_to_kernel_p4dp(p4d_t *p4dp)
+{
+ return ptr_clear_bit(p4dp, KPTI_PGTABLE_SWITCH_BIT);
+}
+#endif /* CONFIG_KERNEL_PAGE_TABLE_ISOLATION */
+
+/*
+ * Page table pages are page-aligned. The lower half of the top
+ * level is used for userspace and the top half for the kernel.
+ *
+ * Returns true for parts of the PGD that map userspace and
+ * false for the parts that map the kernel.
+ */
+static inline bool pgdp_maps_userspace(void *__ptr)
+{
+ unsigned long ptr = (unsigned long)__ptr;
+
+ return (ptr & ~PAGE_MASK) < (PAGE_SIZE / 2);
+}
+
+/*
+ * Does this PGD allow access from userspace?
+ */
+static inline bool pgd_userspace_access(pgd_t pgd)
+{
+ return pgd.pgd & _PAGE_USER;
+}
+
+/*
+ * Take a PGD location (pgdp) and a pgd value that needs to be set there.
+ * Populates the user and returns the resulting PGD that must be set in
+ * the kernel copy of the page tables.
+ */
+static inline pgd_t kpti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
+{
+#ifdef CONFIG_KERNEL_PAGE_TABLE_ISOLATION
+ if (!static_cpu_has_bug(X86_BUG_CPU_SECURE_MODE_KPTI))
+ return pgd;
+
+ if (pgd_userspace_access(pgd)) {
+ if (pgdp_maps_userspace(pgdp)) {
+ /*
+ * The user page tables get the full PGD,
+ * accessible from userspace:
+ */
+ kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
+ /*
+ * For the copy of the pgd that the kernel uses,
+ * make it unusable to userspace. This ensures on
+ * in case that a return to userspace with the
+ * kernel CR3 value, userspace will crash instead
+ * of running.
+ *
+ * Note: NX might be not available or disabled.
+ */
+ if (__supported_pte_mask & _PAGE_NX)
+ pgd.pgd |= _PAGE_NX;
+ }
+ } else if (pgd_userspace_access(*pgdp)) {
+ /*
+ * We are clearing a _PAGE_USER PGD for which we presumably
+ * populated the user PGD. We must now clear the user PGD
+ * entry.
+ */
+ if (pgdp_maps_userspace(pgdp)) {
+ kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
+ } else {
+ /*
+ * Attempted to clear a _PAGE_USER PGD which is in
+ * the kernel porttion of the address space. PGDs
+ * are pre-populated and we never clear them.
+ */
+ WARN_ON_ONCE(1);
+ }
+ } else {
+ /*
+ * _PAGE_USER was not set in either the PGD being set or
+ * cleared. All kernel PGDs should be pre-populated so
+ * this should never happen after boot.
+ */
+ WARN_ON_ONCE(system_state == SYSTEM_RUNNING);
+ }
+#endif
+ /* return the copy of the PGD we want the kernel to use: */
+ return pgd;
+}
+
+
static inline void native_set_p4d(p4d_t *p4dp, p4d_t p4d)
{
+#if defined(CONFIG_KERNEL_PAGE_TABLE_ISOLATION) && !defined(CONFIG_X86_5LEVEL)
+ p4dp->pgd = kpti_set_user_pgd(&p4dp->pgd, p4d.pgd);
+#else
*p4dp = p4d;
+#endif
}

static inline void native_p4d_clear(p4d_t *p4d)
@@ -147,7 +282,11 @@ static inline void native_p4d_clear(p4d_

static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
{
+#ifdef CONFIG_KERNEL_PAGE_TABLE_ISOLATION
+ *pgdp = kpti_set_user_pgd(pgdp, pgd);
+#else
*pgdp = pgd;
+#endif
}

static inline void native_pgd_clear(pgd_t *pgd)

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