The patch below does not apply to the 4.4-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.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

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: [PATCH] s390: always save and restore all registers on context switch

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.

Cc: stable@vger.kernel.org # v2.6.37+ 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

diff --git a/arch/s390/include/asm/switch_to.h b/arch/s390/include/asm/switch_to.h index ec7b476c1ac5..c61b2cc1a8a8 100644 --- 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(unsigned int *acrs) 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 */