linux-kernel - [PATCH 1/3] Revert "sched/x86_64: Don't save flags on context switch"

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Date:	Mon, 17 Aug 2015 12:22:50 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	security@...nel.org, linux-kernel@...r.kernel.org, x86@...nel.org
Cc:	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 1/3] Revert "sched/x86_64: Don't save flags on context switch"

This reverts commit 2c7577a7583747c9b71f26dced7f696b739da745.

It was a nice speedup.  It's also not quite correct: SYSENTER
enables interrupts too early.

We can re-add this optimization once the SYSENTER code is beaten
into shape, which should happen in 4.3 or 4.4.

Cc: stable@...r.kernel.org # v3.19
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/include/asm/switch_to.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 751bf4b7bf11..d7f3b3b78ac3 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -79,12 +79,12 @@ do {									\
 #else /* CONFIG_X86_32 */
 
 /* frame pointer must be last for get_wchan */
-#define SAVE_CONTEXT    "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
-#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
+#define SAVE_CONTEXT    "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t"
+#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t"
 
 #define __EXTRA_CLOBBER  \
 	, "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
-	  "r12", "r13", "r14", "r15", "flags"
+	  "r12", "r13", "r14", "r15"
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 #define __switch_canary							  \
@@ -100,11 +100,7 @@ do {									\
 #define __switch_canary_iparam
 #endif	/* CC_STACKPROTECTOR */
 
-/*
- * There is no need to save or restore flags, because flags are always
- * clean in kernel mode, with the possible exception of IOPL.  Kernel IOPL
- * has no effect.
- */
+/* Save restore flags to clear handle leaking NT */
 #define switch_to(prev, next, last) \
 	asm volatile(SAVE_CONTEXT					  \
 	     "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */	  \
-- 
2.4.3

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