linux-kernel - [PATCH v4 16/57] x86/entry/32: fix the end of the stack for newly forked tasks

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Date:	Thu, 18 Aug 2016 08:05:56 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>,
	Nilay Vaish <nilayvaish@...il.com>
Subject: [PATCH v4 16/57] x86/entry/32: fix the end of the stack for newly forked tasks

The unwinder expects the last frame on the stack to always be at the
same offset from the end of the page, which allows it to validate the
stack.  Calling schedule_tail() directly breaks that convention because
its an asmlinkage function so its argument has to be pushed on the
stack.  Add a wrapper which creates a proper "end of stack" frame header
before the call.

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/entry/entry_32.S | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 0b56666..52b77ac 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -44,6 +44,7 @@
 #include <asm/alternative-asm.h>
 #include <asm/asm.h>
 #include <asm/smap.h>
+#include <asm/frame.h>
 
 	.section .entry.text, "ax"
 
@@ -204,11 +205,27 @@
 	POP_GS_EX
 .endm
 
-ENTRY(ret_from_fork)
+/*
+ * The unwinder expects the last frame on the stack to always be at the same
+ * offset from the end of the page, which allows it to validate the stack.
+ * Calling schedule_tail() directly would break that convention because its an
+ * asmlinkage function so its argument has to be pushed on the stack.  This
+ * wrapper creates a proper "end of stack" frame header before the call.
+ */
+ENTRY(schedule_tail_wrapper)
+	FRAME_BEGIN
+
 	pushl	%eax
 	call	schedule_tail
 	popl	%eax
 
+	FRAME_END
+	ret
+ENDPROC(schedule_tail_wrapper)
+
+ENTRY(ret_from_fork)
+	call	schedule_tail_wrapper
+
 	/* When we fork, we trace the syscall return in the child, too. */
 	movl    %esp, %eax
 	call    syscall_return_slowpath
@@ -216,9 +233,8 @@ ENTRY(ret_from_fork)
 END(ret_from_fork)
 
 ENTRY(ret_from_kernel_thread)
-	pushl	%eax
-	call	schedule_tail
-	popl	%eax
+	call	schedule_tail_wrapper
+
 	movl	PT_EBP(%esp), %eax
 	call	*PT_EBX(%esp)
 	movl	$0, PT_EAX(%esp)
-- 
2.7.4