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.

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

diff --git a/arch/s390/include/asm/switch_to.h b/arch/s390/include/asm/switch_to.h
index dde6b52359c50d8fc4c996bcc7c7ea01813abfb1..ff2fbdafe6899fdee9f433f26dc7a4ce2ed407f6 100644 (file)
--- 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)