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>

2 files changed, 11 insertions, 1 deletions

diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 37a7d2eaf4a..87f9f60b803 100644
--- a/arch/i386/kernel/entry.S
+++ b/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
 
diff --git a/include/asm-i386/system.h b/include/asm-i386/system.h
index 49928eb33f8..098bcee94e3 100644
--- a/include/asm-i386/system.h
+++ b/include/asm-i386/system.h
@@ -11,9 +11,14 @@
 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 */          \
@@ -21,6 +26,7 @@ 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),    \