s390: always save and restore all registers on context switch

commit fbbd7f1a51965b50dd12924841da0d478f3da71b upstream.

The switch_to() macro has an optimization to avoid saving and
restoring register contents that aren't needed for kernel threads.

There is however the possibility that a kernel thread execve's a user
space program. In such a case the execve'd process can partially see
the contents of the previous process, which shouldn't be allowed.

To avoid this, simply always save and restore register contents on
context switch.

Fixes: fdb6d070effba ("switch_to: dont restore/save access & fpu regs for kernel threads")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
[bwh: Backported to 3.2:
 - The save/restore functions are different here
 - FP restore is non-lazy, so drop the comment about it being lazy
 - Adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/s390/include/asm/system.h b/arch/s390/include/asm/system.h
index ef573c1..b68fef3 100644 (file)
--- a/arch/s390/include/asm/system.h
+++ b/arch/s390/include/asm/system.h
@@ -89,17 +89,13 @@ static inline void restore_access_regs(unsigned int *acrs)
        asm volatile("lam 0,15,%0" : : "Q" (*acrs));
 }
 
-#define switch_to(prev,next,last) do {                                 \
-       if (prev->mm) {                                                 \
-               save_fp_regs(&prev->thread.fp_regs);                    \
-               save_access_regs(&prev->thread.acrs[0]);                \
-       }                                                               \
-       if (next->mm) {                                                 \
-               restore_fp_regs(&next->thread.fp_regs);                 \
-               restore_access_regs(&next->thread.acrs[0]);             \
-               update_per_regs(next);                                  \
-       }                                                               \
-       prev = __switch_to(prev,next);                                  \
+#define switch_to(prev, next, last) do {                               \
+       save_fp_regs(&prev->thread.fp_regs);                            \
+       save_access_regs(&prev->thread.acrs[0]);                        \
+       restore_fp_regs(&next->thread.fp_regs);                         \
+       restore_access_regs(&next->thread.acrs[0]);                     \
+       update_per_regs(next);                                          \
+       prev = __switch_to(prev, next);                                 \
 } while (0)
 
 extern void account_vtime(struct task_struct *, struct task_struct *);