[RFC][PATCH v2 06/21] x86/pti: Provide C variants of PTI switch CR3 macros

[Alexandre Chartre]

Page Table Isolation (PTI) use assembly macros to switch the CR3
register between kernel and user page-tables. Add C functions which
implement the same features. For now, these C functions are not
used but they will eventually replace using the assembly macros.

Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
---
 arch/x86/include/asm/entry-common.h | 127 ++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 6fe54b2813c1..46682b1433a4 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -7,6 +7,7 @@
 #include <asm/nospec-branch.h>
 #include <asm/io_bitmap.h>
 #include <asm/fpu/api.h>
+#include <asm/tlbflush.h>
 
 /* Check that the stack and regs on entry from user mode are sane. */
 static __always_inline void arch_check_user_regs(struct pt_regs *regs)
@@ -81,4 +82,130 @@ static __always_inline void arch_exit_to_user_mode(void)
 }
 #define arch_exit_to_user_mode arch_exit_to_user_mode
 
+#ifndef MODULE
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+
+/*
+ * PAGE_TABLE_ISOLATION PGDs are 8k.  Flip bit 12 to switch between the two
+ * halves:
+ */
+#define PTI_USER_PGTABLE_BIT		PAGE_SHIFT
+#define PTI_USER_PGTABLE_MASK		(1 << PTI_USER_PGTABLE_BIT)
+#define PTI_USER_PCID_BIT		X86_CR3_PTI_PCID_USER_BIT
+#define PTI_USER_PCID_MASK		(1 << PTI_USER_PCID_BIT)
+#define PTI_USER_PGTABLE_AND_PCID_MASK  \
+	(PTI_USER_PCID_MASK | PTI_USER_PGTABLE_MASK)
+
+static __always_inline void write_kernel_cr3(unsigned long cr3)
+{
+	if (static_cpu_has(X86_FEATURE_PCID))
+		cr3 |= X86_CR3_PCID_NOFLUSH;
+
+	native_write_cr3(cr3);
+}
+
+static __always_inline void write_user_cr3(unsigned long cr3)
+{
+	unsigned short mask;
+	unsigned long asid;
+
+	if (static_cpu_has(X86_FEATURE_PCID)) {
+		/*
+		 * Test if the ASID needs a flush.
+		 */
+		asid = cr3 & 0x7ff;
+		mask = this_cpu_read(cpu_tlbstate.user_pcid_flush_mask);
+		if (mask & (1 << asid)) {
+			/* Flush needed, clear the bit */
+			this_cpu_and(cpu_tlbstate.user_pcid_flush_mask,
+				     ~(1 << asid));
+		} else {
+			cr3 |= X86_CR3_PCID_NOFLUSH;
+		}
+	}
+
+	native_write_cr3(cr3);
+}
+
+static __always_inline void switch_to_kernel_cr3(unsigned long cr3)
+{
+	/*
+	 * Clear PCID and "PAGE_TABLE_ISOLATION bit", point CR3
+	 * at kernel pagetables.
+	 */
+	write_kernel_cr3(cr3 & ~PTI_USER_PGTABLE_AND_PCID_MASK);
+}
+
+static __always_inline void switch_to_user_cr3(unsigned long cr3)
+{
+	if (static_cpu_has(X86_FEATURE_PCID)) {
+		/* Flip the ASID to the user version */
+		cr3 |= PTI_USER_PCID_MASK;
+	}
+
+	/* Flip the PGD to the user version */
+	write_user_cr3(cr3 | PTI_USER_PGTABLE_MASK);
+}
+
+static __always_inline unsigned long save_and_switch_to_kernel_cr3(void)
+{
+	unsigned long cr3;
+
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return 0;
+
+	cr3 = __native_read_cr3();
+	if (cr3 & PTI_USER_PGTABLE_MASK)
+		switch_to_kernel_cr3(cr3);
+
+	return cr3;
+}
+
+static __always_inline void restore_cr3(unsigned long cr3)
+{
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	if (cr3 & PTI_USER_PGTABLE_MASK) {
+		switch_to_user_cr3(cr3);
+	} else {
+		/*
+		 * The CR3 write could be avoided when not changing
+		 * its value, but would require a CR3 read.
+		 */
+		write_kernel_cr3(cr3);
+	}
+}
+
+static __always_inline void user_pagetable_enter(void)
+{
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	switch_to_user_cr3(__native_read_cr3());
+}
+
+static __always_inline void user_pagetable_exit(void)
+{
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	switch_to_kernel_cr3(__native_read_cr3());
+}
+
+
+#else /* CONFIG_PAGE_TABLE_ISOLATION */
+
+static __always_inline unsigned long save_and_switch_to_kernel_cr3(void)
+{
+	return 0;
+}
+static __always_inline void restore_cr3(unsigned long cr3) {}
+
+static __always_inline void user_pagetable_enter(void) {};
+static __always_inline void user_pagetable_exit(void) {};
+
+#endif /* CONFIG_PAGE_TABLE_ISOLATION */
+#endif /* MODULE */
+
 #endif
-- 
2.18.4