x86: save/restore eflags in context switch

(And reset it on new thread creation)

It turns out that eflags is important to save and restore not just
because of iopl, but due to the magic bits like the NT bit, which we
don't want leaking between different threads.

Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/entry.S

@@ -209,6 +209,10 @@ ENTRY(ret_from_fork)
 	GET_THREAD_INFO(%ebp)
 	popl %eax
 	CFI_ADJUST_CFA_OFFSET -4
+	pushl $0x0202			# Reset kernel eflags
+	CFI_ADJUST_CFA_OFFSET 4
+	popfl
+	CFI_ADJUST_CFA_OFFSET -4
 	jmp syscall_exit
 	CFI_ENDPROC
 

include/asm-i386/system.h

@@ -11,16 +11,22 @@
 struct task_struct;	/* one of the stranger aspects of C forward declarations.. */
 extern struct task_struct * FASTCALL(__switch_to(struct task_struct *prev, struct task_struct *next));
 
+/*
+ * Saving eflags is important. It switches not only IOPL between tasks,
+ * it also protects other tasks from NT leaking through sysenter etc.
+ */
 #define switch_to(prev,next,last) do {					\
 	unsigned long esi,edi;						\
-	asm volatile("pushl %%ebp\n\t"					\
+	asm volatile("pushfl\n\t"		/* Save flags */	\
+		     "pushl %%ebp\n\t"					\
 		     "movl %%esp,%0\n\t"	/* save ESP */		\
 		     "movl %5,%%esp\n\t"	/* restore ESP */	\
 		     "movl $1f,%1\n\t"		/* save EIP */		\
 		     "pushl %6\n\t"		/* restore EIP */	\
 		     "jmp __switch_to\n"				\
 		     "1:\t"						\
 		     "popl %%ebp\n\t"					\
+		     "popfl"						\
 		     :"=m" (prev->thread.esp),"=m" (prev->thread.eip),	\
 		      "=a" (last),"=S" (esi),"=D" (edi)			\
 		     :"m" (next->thread.esp),"m" (next->thread.eip),	\