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%3Ba=su...
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