[PATCH V4 36/50] x86/entry: Implement the C version ist_paranoid_entry()

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

It implements the whole ASM version paranoid_entry().

No functional difference intended.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry64.c        | 37 +++++++++++++++++++++++++++++++++
 arch/x86/include/asm/idtentry.h |  3 +++
 2 files changed, 40 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index 448b9947ed9c..14005122f94b 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -274,3 +274,40 @@ static __always_inline unsigned long ist_switch_to_kernel_gsbase(void)
 	/* SWAPGS required on exit */
 	return 0;
 }
+
+/*
+ * Switch and save CR3 in *@cr3 if PTI enabled. Return GSBASE related
+ * information in *@gsbase depending on the availability of the FSGSBASE
+ * instructions:
+ *
+ * FSGSBASE	*@gsbase
+ *     N        0 -> SWAPGS on exit
+ *              1 -> no SWAPGS on exit
+ *
+ *     Y        GSBASE value at entry, must be restored in ist_paranoid_exit
+ */
+__visible __entry_text
+void ist_paranoid_entry(unsigned long *cr3, unsigned long *gsbase)
+{
+	/*
+	 * Always stash CR3 in *@cr3.  This value will be restored,
+	 * verbatim, at exit.  Needed if ist_paranoid_entry interrupted
+	 * another entry that already switched to the user CR3 value
+	 * but has not yet returned to userspace.
+	 *
+	 * This is also why CS (stashed in the "iret frame" by the
+	 * hardware at entry) can not be used: this may be a return
+	 * to kernel code, but with a user CR3 value.
+	 *
+	 * Switching CR3 does not depend on kernel GSBASE so it can
+	 * be done before switching to the kernel GSBASE. This is
+	 * required for FSGSBASE because the kernel GSBASE has to
+	 * be retrieved from a kernel internal table.
+	 */
+	*cr3 = ist_switch_to_kernel_cr3();
+
+	barrier();
+
+	/* Handle GSBASE, store the return value in *@gsbase for exit. */
+	*gsbase = ist_switch_to_kernel_gsbase();
+}
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 49fabc3e3f0d..f6efa21ec242 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -307,6 +307,9 @@ static __always_inline void __##func(struct pt_regs *regs)
 	DECLARE_IDTENTRY(vector, func)
 
 #ifdef CONFIG_X86_64
+__visible __entry_text
+void ist_paranoid_entry(unsigned long *cr3, unsigned long *gsbase);
+
 /**
  * DECLARE_IDTENTRY_IST - Declare functions for IST handling IDT entry points
  * @vector:	Vector number (ignored for C)
-- 
2.19.1.6.gb485710b