[PATCH] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code

[Andy Lutomirski <luto@kernel.org>]

The error_entry/error_exit code to handle gsbase and whether we return
to user mdoe was a mess:

 - error_sti was misnamed.  In particular, it did not enable interrupts.

 - Error handling for gs_change was hopelessly tangled the normal
   usermode path.  Separate it out.  This saves a branch in normal
   entries from kernel mode.

 - The comments were bad.

Fix it up.  As a nice side effect, there's now a code path that happens
on error entries from user mode.  We'll use it soon.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64.S | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index bd97161f90cb..3462a4f09425 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1135,7 +1135,7 @@ END(paranoid_exit)
 
 /*
  * Save all registers in pt_regs, and switch gs if needed.
- * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
+ * Return: ebx=0: came from user mode; ebx=1: otherwise
  */
 ENTRY(error_entry)
 	cld
@@ -1144,9 +1144,11 @@ ENTRY(error_entry)
 	xorl	%ebx, %ebx
 	testb	$3, CS+8(%rsp)
 	jz	error_kernelspace
-error_swapgs:
+
+	/* We entered from user mode */
 	SWAPGS
-error_sti:
+
+error_entry_done:
 	TRACE_IRQS_OFF
 	ret
 
@@ -1165,8 +1167,15 @@ error_kernelspace:
 	cmpq	%rax, RIP+8(%rsp)
 	je	bstep_iret
 	cmpq	$gs_change, RIP+8(%rsp)
-	je	error_swapgs
-	jmp	error_sti
+	jne	error_entry_done
+
+	/*
+	 * hack: gs_change can fail with user gsbase.  If this happens, fix up
+	 * gsbase and proceed.  We'll fix up the exception and land in
+	 * gs_change's error handler with kernel gsbase.
+	 */
+	SWAPGS
+	jmp	error_entry_done
 
 bstep_iret:
 	/* Fix truncated RIP */
@@ -1174,16 +1183,30 @@ bstep_iret:
 	/* fall through */
 
 error_bad_iret:
+	/*
+	 * We came from an IRET to user mode, so we have user gsbase.
+	 * Switch to kernel gsbase.
+	 */
 	SWAPGS
+
+	/*
+	 * Pretend that the exception came from user mode: set up pt_regs
+	 * as if we faulted immediately after IRET and zero ebx so that
+	 * error_exit knows that we will returning to user mode.
+	 */
 	mov	%rsp, %rdi
 	call	fixup_bad_iret
 	mov	%rax, %rsp
-	decl	%ebx				/* Return to usergs */
-	jmp	error_sti
+	decl	%ebx
+	jmp	error_entry_done
 END(error_entry)
 
 
-/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
+/*
+ * On entry, ebx is "return to kernel" flag
+ * 1: don't need swapgs
+ * 0: need swapgs and standard preparation for return to usermode
+ */
 ENTRY(error_exit)
 	movl	%ebx, %eax
 	RESTORE_EXTRA_REGS
-- 
2.4.2