From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: s390/entry.S: fix spurious zeroing of r0
Patch-mainline: v4.16-rc5
Git-commit: d3f468963cd6fd6d2aa5e26aed8b24232096d0e1
References: LTC#164304, bsc#1084911

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 zero out 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>
Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 arch/s390/kernel/entry.S |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -424,13 +424,13 @@ 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)