Re: FAILED: patch "[PATCH] s390: always save and restore all registers on context switch" failed to apply to 4.9-stable tree

[Heiko Carstens <heiko.carstens@de.ibm.com>]

On Mon, Dec 11, 2017 at 10:30:29PM +0100, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 4.9-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

Please find below the patch applies against 4.9-stable.

>From ac37e8353f3ef8330ddb9376c316021c8be0776f 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: [PATCH] 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>
---
 arch/s390/include/asm/switch_to.h | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/arch/s390/include/asm/switch_to.h b/arch/s390/include/asm/switch_to.h
index dde6b52359c5..ff2fbdafe689 100644
--- a/arch/s390/include/asm/switch_to.h
+++ b/arch/s390/include/asm/switch_to.h
@@ -29,17 +29,16 @@ static inline void restore_access_regs(unsigned int *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_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);					\
 	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_access_regs(&next->thread.acrs[0]);			\
+	restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);		\
 	prev = __switch_to(prev,next);					\
 } while (0)
 
-- 
2.13.5