x86/entry/64: Switch CR3 before SWAPGS in paranoid entry

When FSGSBASE is enabled, the GSBASE handling in paranoid entry will need
to retrieve the kernel GSBASE which requires that the kernel page table is
active.

As the CR3 switch to the kernel page tables (PTI is active) does not depend
on kernel GSBASE, move the CR3 switch in front of the GSBASE handling.

Comment the EBX content while at it.

No functional change.

[ tglx: Rewrote changelog and comments ]
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1557309753-24073-11-git-send-email-chang.seok.bae@intel.com
Link: https://lkml.kernel.org/r/20200528201402.1708239-10-sashal@kernel.org

arch/x86/entry/entry_64.S

@@ -807,13 +807,6 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	cld
 	PUSH_AND_CLEAR_REGS save_ret=1
 	ENCODE_FRAME_POINTER 8
-	movl	$1, %ebx
-	movl	$MSR_GS_BASE, %ecx
-	rdmsr
-	testl	%edx, %edx
-	js	1f				/* negative -> in kernel */
-	SWAPGS
-	xorl	%ebx, %ebx
 
 1:
 	/*
@@ -825,9 +818,29 @@ SYM_CODE_START_LOCAL(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.
+	 *
+	 * 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.
 	 */
 	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
 
+	/* EBX = 1 -> kernel GSBASE active, no restore required */
+	movl	$1, %ebx
+	/*
+	 * The kernel-enforced convention is a negative GSBASE indicates
+	 * a kernel value. No SWAPGS needed on entry and exit.
+	 */
+	movl	$MSR_GS_BASE, %ecx
+	rdmsr
+	testl	%edx, %edx
+	jns	.Lparanoid_entry_swapgs
+	ret
+
+.Lparanoid_entry_swapgs:
+	SWAPGS
+
 	/*
 	 * The above SAVE_AND_SWITCH_TO_KERNEL_CR3 macro doesn't do an
 	 * unconditional CR3 write, even in the PTI case.  So do an lfence
@@ -835,6 +848,8 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	 */
 	FENCE_SWAPGS_KERNEL_ENTRY
 
+	/* EBX = 0 -> SWAPGS required on exit */
+	xorl	%ebx, %ebx
 	ret
 SYM_CODE_END(paranoid_entry)
 
@@ -852,7 +867,8 @@ SYM_CODE_END(paranoid_entry)
  */
 SYM_CODE_START_LOCAL(paranoid_exit)
 	UNWIND_HINT_REGS
-	testl	%ebx, %ebx			/* swapgs needed? */
+	/* If EBX is 0, SWAPGS is required */
+	testl	%ebx, %ebx
 	jnz	.Lparanoid_exit_no_swapgs
 	/* Always restore stashed CR3 value (see paranoid_entry) */
 	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14