[PATCH 3.16 123/157] x86/entry/64: Really create an error-entry-from-usermode code path

[Ben Hutchings]

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

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

From: Andy Lutomirski <luto@xxxxxxxxxx>

commit cb6f64ed5a04036eef07e70b57dd5dd78f2fbcef upstream.

In 539f51136500 ("x86/asm/entry/64: Disentangle error_entry/exit
gsbase/ebx/usermode code"), I arranged the code slightly wrong
-- IRET faults would skip the code path that was intended to
execute on all error entries from user mode.  Fix it up.

While we're at it, make all the labels in error_entry local.

This does not fix a bug, but we'll need it, and it slightly
shrinks the code.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: paulmck@xxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/91e17891e49fa3d61357eadc451529ad48143ee1.1435952415.git.luto@xxxxxxxxxx
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":
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/entry_64.S | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1445,12 +1445,17 @@ ENTRY(error_entry)
 	 */
 	SWITCH_KERNEL_CR3
 	testl $3,CS+8(%rsp)
-	je error_kernelspace
+	jz	.Lerror_kernelspace
 
-	/* We entered from user mode */
+.Lerror_entry_from_usermode_swapgs:
+	/*
+	 * We entered from user mode or we're pretending to have entered
+	 * from user mode due to an IRET fault.
+	 */
 	SWAPGS
 
-error_entry_done:
+.Lerror_entry_from_usermode_after_swapgs:
+.Lerror_entry_done:
 	TRACE_IRQS_OFF
 	ret
 
@@ -1460,30 +1465,29 @@ error_entry_done:
  * truncated RIP for IRET exceptions returning to compat mode. Check
  * for these here too.
  */
-error_kernelspace:
+.Lerror_kernelspace:
 	leaq native_irq_return_iret(%rip),%rcx
 	cmpq %rcx,RIP+8(%rsp)
-	je error_bad_iret
+	je	.Lerror_bad_iret
 	movl %ecx,%eax	/* zero extend */
 	cmpq %rax,RIP+8(%rsp)
-	je bstep_iret
+	je	.Lbstep_iret
 	cmpq $gs_change,RIP+8(%rsp)
-	jne	error_entry_done
+	jne	.Lerror_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
+	jmp	.Lerror_entry_from_usermode_swapgs
 
-bstep_iret:
+.Lbstep_iret:
 	/* Fix truncated RIP */
 	movq %rcx,RIP+8(%rsp)
 	/* fall through */
 
-error_bad_iret:
+.Lerror_bad_iret:
 	/*
 	 * We came from an IRET to user mode, so we have user gsbase.
 	 * Switch to kernel gsbase:
@@ -1497,7 +1501,7 @@ error_bad_iret:
 	mov %rsp,%rdi
 	call fixup_bad_iret
 	mov %rax,%rsp
-	jmp	error_entry_done
+	jmp	.Lerror_entry_from_usermode_after_swapgs
 	CFI_ENDPROC
 END(error_entry)