Patch "s390: always save and restore all registers on context switch" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

s390: always save and restore all registers on context switch

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
s390-always-save-and-restore-all-registers-on-context-switch.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Heiko Carstens <[email protected]>
Date: Mon, 20 Nov 2017 12:38:44 +0100
Subject: s390: always save and restore all registers on context switch

From: Heiko Carstens <[email protected]>

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.

---
 arch/s390/include/asm/switch_to.h |   27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

@@ -30,21 +30,20 @@ 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_fpu_regs();					\
-		save_access_regs(&prev->thread.acrs[0]);		\
-		save_ri_cb(prev->thread.ri_cb);				\
-		save_gs_cb(prev->thread.gs_cb);				\
-	}								\
+#define switch_to(prev, next, last) do {				\
+	/* save_fpu_regs() sets the CIF_FPU flag, which enforces	\
+	 * a restore of the floating point / vector registers as	\
+	 * soon as the next task returns to user space			\
+	 */								\
+	save_fpu_regs();						\
+	save_access_regs(&prev->thread.acrs[0]);			\
+	save_ri_cb(prev->thread.ri_cb);					\
+	save_gs_cb(prev->thread.gs_cb);					\
 	update_cr_regs(next);						\
-	if (next->mm) {							\
-		set_cpu_flag(CIF_FPU);					\
-		restore_access_regs(&next->thread.acrs[0]);		\
-		restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);	\
-		restore_gs_cb(next->thread.gs_cb);			\
-	}								\
-	prev = __switch_to(prev,next);					\
+	restore_access_regs(&next->thread.acrs[0]);			\
+	restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);		\
+	restore_gs_cb(next->thread.gs_cb);				\
+	prev = __switch_to(prev, next);					\
 } while (0)
 
 #endif /* __ASM_SWITCH_TO_H */

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/s390-always-save-and-restore-all-registers-on-context-switch.patch queue-4.14/s390-fix-compat-system-call-table.patch queue-4.14/s390-mm-fix-off-by-one-bug-in-5-level-page-table-handling.patch