i386: save/restore eflags in context switch (CVE-2006-5173)

(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.

Backported to 2.6.16 by Chuck Ebbert <76306.1226@compuserve.com>
[Backport consisted of removing the CFI annotations.]
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

arch/i386/kernel/entry.S

@@ -128,6 +128,8 @@ ENTRY(ret_from_fork)
 	call schedule_tail
 	GET_THREAD_INFO(%ebp)
 	popl %eax
+	pushl $0x0202			# Reset kernel eflags
+	popfl
 	jmp syscall_exit
 
 /*

include/asm-i386/system.h

@@ -12,9 +12,14 @@
 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 */		\
@@ -22,6 +27,7 @@ extern struct task_struct * FASTCALL(__switch_to(struct task_struct *prev, struc
 		     "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),	\