[PATCH 3.16 122/157] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code

[Ben Hutchings]

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@xxxxxxxxxx>

commit 539f5113650068ba221197f190267ab727296ef5 upstream.

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@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/f1be898ab93360169fb845ab85185948832209ee.1433878454.git.luto@xxxxxxxxxx
[ Prettified it, clarified comments some more. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.16 as dependency of commit 18ec54fdd6d1
 "x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations":
 - We do not use %ebx as a flag since we already have a backport of commit
   b3681dd548d0 "x86/entry/64: Remove %ebx handling from error_entry/exit",
   so don't add the comments about that
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1446,9 +1446,11 @@ ENTRY(error_entry)
 	SWITCH_KERNEL_CR3
 	testl $3,CS+8(%rsp)
 	je error_kernelspace
-error_swapgs:
+
+	/* We entered from user mode */
 	SWAPGS
-error_sti:
+
+error_entry_done:
 	TRACE_IRQS_OFF
 	ret
 
@@ -1466,8 +1468,15 @@ 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 */
@@ -1475,11 +1484,20 @@ 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.
+	 */
 	mov %rsp,%rdi
 	call fixup_bad_iret
 	mov %rax,%rsp
-	jmp error_sti
+	jmp	error_entry_done
 	CFI_ENDPROC
 END(error_entry)