4.9-stable review patch. If anyone has any objections, please let me know.
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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 | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-)
--- 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(u }
#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)