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

[<gregkh@linuxfoundation.org>]

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 <stable@vger.kernel.org> know about it.


>From fbbd7f1a51965b50dd12924841da0d478f3da71b Mon Sep 17 00:00:00 2001
From: Heiko Carstens <heiko.carstens@de.ibm.com>
Date: Mon, 20 Nov 2017 12:38:44 +0100
Subject: s390: always save and restore all registers on context switch

From: Heiko Carstens <heiko.carstens@de.ibm.com>

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/s390/include/asm/switch_to.h
+++ b/arch/s390/include/asm/switch_to.h
@@ -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 heiko.carstens@de.ibm.com 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