[PATCH v7 10/18] x86/entry/64: Switch CR3 before SWAPGS on the paranoid entry

[Chang S. Bae]

When FSGSBASE is enabled, GSBASE handling on the paranoid entry will
need to retrieve the kernel GSBASE. Thus, the kernel page table should
be in. As a preparation, the CR3 switching is moved to happen at
first, before the SWAPGS.

Current GSBASE switching mechanism is possible without the kernel
page table in. No functional change is expected.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 568a491..034d8f8 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1160,18 +1160,6 @@ ENTRY(paranoid_entry)
 	PUSH_AND_CLEAR_REGS save_ret=1
 	ENCODE_FRAME_POINTER 8
 
-	movl	$1, %ebx
-	/*
-	 * The kernel-enforced convention is a negative GSBASE indicates
-	 * a kernel value.
-	 */
-	READ_MSR_GSBASE save_reg=%edx
-	testl	%edx, %edx	/* Negative -> in kernel */
-	js	1f
-	SWAPGS
-	xorl	%ebx, %ebx
-
-1:
 	/*
 	 * Always stash CR3 in %r14.  This value will be restored,
 	 * verbatim, at exit.  Needed if paranoid_entry interrupted
@@ -1181,9 +1169,26 @@ ENTRY(paranoid_entry)
 	 * 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.
+	 *
+	 * This PTI macro doesn't depend on kernel GSBASE and, with
+	 * FSGSBASE, the GSBASE handling requires the kernel page
+	 * tables switched in. So, do it early here.
 	 */
 	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
 
+	movl	$1, %ebx
+	/*
+	 * The kernel-enforced convention is a negative GSBASE indicates
+	 * a kernel value.
+	 */
+	READ_MSR_GSBASE save_reg=%edx
+	testl	%edx, %edx	/* Negative -> in kernel */
+	jns	.Lparanoid_entry_swapgs
+	ret
+
+.Lparanoid_entry_swapgs:
+	SWAPGS
+	xorl	%ebx, %ebx
 	ret
 END(paranoid_entry)
 
-- 
2.7.4