linux-kernel - [PATCH v3 08/15] x86/entry/64: Really create an error-entry-from-usermode code path

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Date:	Tue, 23 Jun 2015 19:46:11 -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 v3 08/15] 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.

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 | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 3bb2c4302df1..b47f9686dece 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1145,9 +1145,14 @@ ENTRY(error_entry)
 	testb	$3, CS+8(%rsp)
 	jz	error_kernelspace
 
-	/* We entered from user mode */
+error_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_from_usermode_after_swapgs:
 error_entry_done:
 	TRACE_IRQS_OFF
 	ret
@@ -1174,8 +1179,7 @@ error_kernelspace:
 	 * 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	error_entry_from_usermode_swapgs
 
 bstep_iret:
 	/* Fix truncated RIP */
@@ -1198,7 +1202,7 @@ error_bad_iret:
 	call	fixup_bad_iret
 	mov	%rax, %rsp
 	decl	%ebx
-	jmp	error_entry_done
+	jmp	error_entry_from_usermode_after_swapgs
 END(error_entry)
 
 
-- 
2.4.3

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