[PATCH v9 04/17] x86/entry/64: Clean up paranoid exit

[Chang S. Bae]

From: Andy Lutomirski <luto@xxxxxxxxxx>

All that paranoid exit needs to do is to disable IRQs, handle IRQ tracing,
then restore CR3, and restore GS base. Simply do those actions in that
order. Cleaning up the spaghetti code.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
---

Changes from v8: none

Changes from v7:
* Included as a new patch. Took the cleanup part from the Andy Lutomirski's
  original patch [*] and edited its changelog a little bit.

[*] https://lkml.kernel.org/r/59725ceb08977359489fbed979716949ad45f616.1562035429.git.luto@xxxxxxxxxx
---
 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 b7c3ea4..dd0d62a 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1265,20 +1265,25 @@ END(paranoid_entry)
 ENTRY(paranoid_exit)
 	UNWIND_HINT_REGS
 	DISABLE_INTERRUPTS(CLBR_ANY)
+
+	/*
+	 * The order of operations is important. IRQ tracing requires
+	 * kernel GS base and CR3. RESTORE_CR3 requires kernel GS base.
+	 *
+	 * NB to anyone to try to optimize this code: this code does
+	 * not execute at all for exceptions from user mode. Those
+	 * exceptions go through error_exit instead.
+	 */
 	TRACE_IRQS_OFF_DEBUG
-	testl	%ebx, %ebx			/* swapgs needed? */
-	jnz	.Lparanoid_exit_no_swapgs
-	TRACE_IRQS_IRETQ
-	/* Always restore stashed CR3 value (see paranoid_entry) */
-	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
+	RESTORE_CR3	scratch_reg=%rax save_reg=%r14
+
+	/* If EBX is 0, SWAPGS is required */
+	testl	%ebx, %ebx
+	jnz	restore_regs_and_return_to_kernel
+
+	/* We are returning to a context with user GS base */
 	SWAPGS_UNSAFE_STACK
-	jmp	.Lparanoid_exit_restore
-.Lparanoid_exit_no_swapgs:
-	TRACE_IRQS_IRETQ_DEBUG
-	/* Always restore stashed CR3 value (see paranoid_entry) */
-	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
-.Lparanoid_exit_restore:
-	jmp restore_regs_and_return_to_kernel
+	jmp	restore_regs_and_return_to_kernel
 END(paranoid_exit)
 
 /*
-- 
2.7.4