[PATCH 13/35] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

Add unconditional cr3 switches between user and kernel cr3
to all non-NMI entry and exit points.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/entry/entry_32.S | 83 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 79 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 3c4822a..86b3fe6d 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -154,6 +154,33 @@
 
 #endif /* CONFIG_X86_32_LAZY_GS */
 
+/* Unconditionally switch to user cr3 */
+.macro SWITCH_TO_USER_CR3 scratch_reg:req
+	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
+
+	movl	%cr3, \scratch_reg
+	orl	$PTI_SWITCH_MASK, \scratch_reg
+	movl	\scratch_reg, %cr3
+.Lend_\@:
+.endm
+
+/*
+ * Switch to kernel cr3 if not already loaded and return current cr3 in
+ * \scratch_reg
+ */
+.macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
+	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
+	movl	%cr3, \scratch_reg
+	/* Test if we are already on kernel CR3 */
+	testl	$PTI_SWITCH_MASK, \scratch_reg
+	jz	.Lend_\@
+	andl	$(~PTI_SWITCH_MASK), \scratch_reg
+	movl	\scratch_reg, %cr3
+	/* Return original CR3 in \scratch_reg */
+	orl	$PTI_SWITCH_MASK, \scratch_reg
+.Lend_\@:
+.endm
+
 .macro SAVE_ALL pt_regs_ax=%eax switch_stacks=0
 	cld
 	/* Push segment registers and %eax */
@@ -288,7 +315,6 @@
 #endif /* CONFIG_X86_ESPFIX32 */
 .endm
 
-
 /*
  * Called with pt_regs fully populated and kernel segments loaded,
  * so we can access PER_CPU and use the integer registers.
@@ -301,11 +327,19 @@
  */
 
 #define CS_FROM_ENTRY_STACK	(1 << 31)
+#define CS_FROM_USER_CR3	(1 << 30)
 
 .macro SWITCH_TO_KERNEL_STACK
 
 	ALTERNATIVE     "", "jmp .Lend_\@", X86_FEATURE_XENPV
 
+	SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
+
+	/*
+	 * %eax now contains the entry cr3 and we carry it forward in
+	 * that register for the time this macro runs
+	 */
+
 	/* Are we on the entry stack? Bail out if not! */
 	movl	PER_CPU_VAR(cpu_entry_area), %edi
 	addl	$CPU_ENTRY_AREA_entry_stack, %edi
@@ -374,7 +408,8 @@
 	 * but switch back to the entry-stack again when we approach
 	 * iret and return to the interrupted code-path. This usually
 	 * happens when we hit an exception while restoring user-space
-	 * segment registers on the way back to user-space.
+	 * segment registers on the way back to user-space or when the
+	 * sysenter handler runs with eflags.tf set.
 	 *
 	 * When we switch to the task-stack here, we can't trust the
 	 * contents of the entry-stack anymore, as the exception handler
@@ -391,6 +426,7 @@
 	 *
 	 * %esi: Entry-Stack pointer (same as %esp)
 	 * %edi: Top of the task stack
+	 * %eax: CR3 on kernel entry
 	 */
 
 	/* Calculate number of bytes on the entry stack in %ecx */
@@ -407,6 +443,14 @@
 	orl	$CS_FROM_ENTRY_STACK, PT_CS(%esp)
 
 	/*
+	 * Test the cr3 used to enter the kernel and add a marker
+	 * so that we can switch back to it before iret.
+	 */
+	testl	$PTI_SWITCH_MASK, %eax
+	jz	.Lcopy_pt_regs_\@
+	orl	$CS_FROM_USER_CR3, PT_CS(%esp)
+
+	/*
 	 * %esi and %edi are unchanged, %ecx contains the number of
 	 * bytes to copy. The code at .Lcopy_pt_regs_\@ will allocate
 	 * the stack-frame on task-stack and copy everything over
@@ -472,7 +516,7 @@
 
 /*
  * This macro handles the case when we return to kernel-mode on the iret
- * path and have to switch back to the entry stack.
+ * path and have to switch back to the entry stack and/or user-cr3
  *
  * See the comments below the .Lentry_from_kernel_\@ label in the
  * SWITCH_TO_KERNEL_STACK macro for more details.
@@ -518,6 +562,18 @@
 	/* Safe to switch to entry-stack now */
 	movl	%ebx, %esp
 
+	/*
+	 * We came from entry-stack and need to check if we also need to
+	 * switch back to user cr3.
+	 */
+	testl	$CS_FROM_USER_CR3, PT_CS(%esp)
+	jz	.Lend_\@
+
+	/* Clear marker from stack-frame */
+	andl	$(~CS_FROM_USER_CR3), PT_CS(%esp)
+
+	SWITCH_TO_USER_CR3 scratch_reg=%eax
+
 .Lend_\@:
 .endm
 /*
@@ -712,6 +768,18 @@ ENTRY(xen_sysenter_target)
  * 0(%ebp) arg6
  */
 ENTRY(entry_SYSENTER_32)
+	/*
+	 * On entry-stack with all userspace-regs live - save and
+	 * restore eflags and %eax to use it as scratch-reg for the cr3
+	 * switch.
+	 */
+	pushfl
+	pushl	%eax
+	SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
+	popl	%eax
+	popfl
+
+	/* Stack empty again, switch to task stack */
 	movl	TSS_entry_stack(%esp), %esp
 
 .Lsysenter_past_esp:
@@ -792,6 +860,9 @@ ENTRY(entry_SYSENTER_32)
 	/* Switch to entry stack */
 	movl	%eax, %esp
 
+	/* Now ready to switch the cr3 */
+	SWITCH_TO_USER_CR3 scratch_reg=%eax
+
 	/*
 	 * Restore all flags except IF. (We restore IF separately because
 	 * STI gives a one-instruction window in which we won't be interrupted,
@@ -872,7 +943,11 @@ restore_all:
 .Lrestore_all_notrace:
 	CHECK_AND_APPLY_ESPFIX
 .Lrestore_nocheck:
-	RESTORE_REGS 4				# skip orig_eax/error_code
+	/* Switch back to user CR3 */
+	SWITCH_TO_USER_CR3 scratch_reg=%eax
+
+	/* Restore user state */
+	RESTORE_REGS pop=4			# skip orig_eax/error_code
 .Lirq_return:
 	INTERRUPT_RETURN
 
-- 
2.7.4