[PATCH 3.16 055/254] s390: always save and restore all registers on context switch

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
[bwh: Backported to 3.16:
 - The save/restore functions are different here
 - FP restore is non-lazy, so drop the comment about it being lazy]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/s390/include/asm/switch_to.h
+++ b/arch/s390/include/asm/switch_to.h
@@ -117,21 +117,17 @@ static inline void restore_access_regs(u
 	asm volatile("lam 0,15,%0" : : "Q" (*(acrstype *)acrs));
 }
 
-#define switch_to(prev,next,last) do {					\
-	if (prev->mm) {							\
-		save_fp_ctl(&prev->thread.fp_regs.fpc);			\
-		save_fp_regs(prev->thread.fp_regs.fprs);		\
-		save_access_regs(&prev->thread.acrs[0]);		\
-		save_ri_cb(prev->thread.ri_cb);				\
-	}								\
+#define switch_to(prev, next, last) do {				\
+	save_fp_ctl(&prev->thread.fp_regs.fpc);				\
+	save_fp_regs(prev->thread.fp_regs.fprs);			\
+	save_access_regs(&prev->thread.acrs[0]);			\
+	save_ri_cb(prev->thread.ri_cb);					\
 	update_cr_regs(next);						\
-	if (next->mm) {							\
-		restore_fp_ctl(&next->thread.fp_regs.fpc);		\
-		restore_fp_regs(next->thread.fp_regs.fprs);		\
-		restore_access_regs(&next->thread.acrs[0]);		\
-		restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);	\
-	}								\
-	prev = __switch_to(prev,next);					\
+	restore_fp_ctl(&next->thread.fp_regs.fpc);			\
+	restore_fp_regs(next->thread.fp_regs.fprs);			\
+	restore_access_regs(&next->thread.acrs[0]);			\
+	restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);		\
+	prev = __switch_to(prev, next);					\
 } while (0)
 
 #endif /* __ASM_SWITCH_TO_H */