s390/entry.S: fix spurious zeroing of r0

when a system call is interrupted we might call the critical section
cleanup handler that re-does some of the operations. When we are between
.Lsysc_vtime and .Lsysc_do_svc we might also redo the saving of the
problem state registers r0-r7:

.Lcleanup_system_call:
[...]
0:      # update accounting time stamp
        mvc     __LC_LAST_UPDATE_TIMER(8),__LC_SYNC_ENTER_TIMER
        # set up saved register r11
        lg      %r15,__LC_KERNEL_STACK
        la      %r9,STACK_FRAME_OVERHEAD(%r15)
        stg     %r9,24(%r11)            # r11 pt_regs pointer
        # fill pt_regs
        mvc     __PT_R8(64,%r9),__LC_SAVE_AREA_SYNC
--->    stmg    %r0,%r7,__PT_R0(%r9)

The problem is now, that we might have already zeroed out r0.
The fix is to move the zeroing of r0 after sysc_do_svc.

Reported-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Fixes: 7041d28115e91 ("s390: scrub registers on kernel entry and KVM exit")
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 73492461c45493618ffcd36c524f93fa85800369..a5621ea6d1234f1c3b6c8aec8612bd933f5cc6f3 100644 (file)
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -427,13 +427,13 @@ ENTRY(system_call)
        UPDATE_VTIME %r8,%r9,__LC_SYNC_ENTER_TIMER
        BPENTER __TI_flags(%r12),_TIF_ISOLATE_BP
        stmg    %r0,%r7,__PT_R0(%r11)
-       # clear user controlled register to prevent speculative use
-       xgr     %r0,%r0
        mvc     __PT_R8(64,%r11),__LC_SAVE_AREA_SYNC
        mvc     __PT_PSW(16,%r11),__LC_SVC_OLD_PSW
        mvc     __PT_INT_CODE(4,%r11),__LC_SVC_ILC
        stg     %r14,__PT_FLAGS(%r11)
 .Lsysc_do_svc:
+       # clear user controlled register to prevent speculative use
+       xgr     %r0,%r0
        # load address of system call table
        lg      %r10,__THREAD_sysc_table(%r13,%r12)
        llgh    %r8,__PT_INT_CODE+2(%r11)