linux-kernel - [PATCH v5 10/17] x86/entry/64: Really create an error-entry-from-usermode code path

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Date:	Fri,  3 Jul 2015 12:44:27 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>, paulmck@...ux.vnet.ibm.com,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v5 10/17] x86/entry/64: Really create an error-entry-from-usermode code path

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@...nel.org>
---
 arch/x86/entry/entry_64.S | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 141a5d49dddc..ccfcba90de6e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1143,12 +1143,17 @@ ENTRY(error_entry)
 	SAVE_EXTRA_REGS 8
 	xorl	%ebx, %ebx
 	testb	$3, CS+8(%rsp)
-	jz	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
 
@@ -1158,31 +1163,30 @@ error_entry_done:
 	 * truncated RIP for IRET exceptions returning to compat mode. Check
 	 * for these here too.
 	 */
-error_kernelspace:
+.Lerror_kernelspace:
 	incl	%ebx
 	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:
@@ -1198,7 +1202,7 @@ error_bad_iret:
 	call	fixup_bad_iret
 	mov	%rax, %rsp
 	decl	%ebx
-	jmp	error_entry_done
+	jmp	.Lerror_entry_from_usermode_after_swapgs
 END(error_entry)
 
 
-- 
2.4.3

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