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From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org
Cc:     Heiko Carstens <heiko.carstens@de.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jon Masters <jcm@redhat.com>,
        Marcus Meissner <meissner@suse.de>,
        Jiri Kosina <jkosina@suse.cz>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Pavel Machek <pavel@ucw.cz>,
        David Woodhouse <dwmw2@infradead.org>
Subject: [PATCH 1/6] s390: scrub registers on kernel entry and KVM exit
Date:   Wed,  7 Feb 2018 08:00:06 +0100
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Clear all user space registers on entry to the kernel and all KVM guest
registers on KVM guest exit if the register does not contain either a
parameter or a result value.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
 arch/s390/kernel/entry.S | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 6cd444d..5d87eda 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -248,6 +248,12 @@ ENTRY(sie64a)
 sie_exit:
 	lg	%r14,__SF_EMPTY+8(%r15)		# load guest register save area
 	stmg	%r0,%r13,0(%r14)		# save guest gprs 0-13
+	xgr	%r0,%r0				# clear guest registers to
+	xgr	%r1,%r1				# prevent speculative use
+	xgr	%r2,%r2
+	xgr	%r3,%r3
+	xgr	%r4,%r4
+	xgr	%r5,%r5
 	lmg	%r6,%r14,__SF_GPRS(%r15)	# restore kernel registers
 	lg	%r2,__SF_EMPTY+16(%r15)		# return exit reason code
 	br	%r14
@@ -282,6 +288,8 @@ ENTRY(system_call)
 .Lsysc_vtime:
 	UPDATE_VTIME %r8,%r9,__LC_SYNC_ENTER_TIMER
 	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
@@ -561,6 +569,15 @@ ENTRY(pgm_check_handler)
 4:	lgr	%r13,%r11
 	la	%r11,STACK_FRAME_OVERHEAD(%r15)
 	stmg	%r0,%r7,__PT_R0(%r11)
+	# clear user controlled registers to prevent speculative use
+	xgr	%r0,%r0
+	xgr	%r1,%r1
+	xgr	%r2,%r2
+	xgr	%r3,%r3
+	xgr	%r4,%r4
+	xgr	%r5,%r5
+	xgr	%r6,%r6
+	xgr	%r7,%r7
 	mvc	__PT_R8(64,%r11),__LC_SAVE_AREA_SYNC
 	stmg	%r8,%r9,__PT_PSW(%r11)
 	mvc	__PT_INT_CODE(4,%r11),__LC_PGM_ILC
@@ -626,6 +643,16 @@ ENTRY(io_int_handler)
 	lmg	%r8,%r9,__LC_IO_OLD_PSW
 	SWITCH_ASYNC __LC_SAVE_AREA_ASYNC,__LC_ASYNC_ENTER_TIMER
 	stmg	%r0,%r7,__PT_R0(%r11)
+	# clear user controlled registers to prevent speculative use
+	xgr	%r0,%r0
+	xgr	%r1,%r1
+	xgr	%r2,%r2
+	xgr	%r3,%r3
+	xgr	%r4,%r4
+	xgr	%r5,%r5
+	xgr	%r6,%r6
+	xgr	%r7,%r7
+	xgr	%r10,%r10
 	mvc	__PT_R8(64,%r11),__LC_SAVE_AREA_ASYNC
 	stmg	%r8,%r9,__PT_PSW(%r11)
 	mvc	__PT_INT_CODE(12,%r11),__LC_SUBCHANNEL_ID
@@ -839,6 +866,16 @@ ENTRY(ext_int_handler)
 	lmg	%r8,%r9,__LC_EXT_OLD_PSW
 	SWITCH_ASYNC __LC_SAVE_AREA_ASYNC,__LC_ASYNC_ENTER_TIMER
 	stmg	%r0,%r7,__PT_R0(%r11)
+	# clear user controlled registers to prevent speculative use
+	xgr	%r0,%r0
+	xgr	%r1,%r1
+	xgr	%r2,%r2
+	xgr	%r3,%r3
+	xgr	%r4,%r4
+	xgr	%r5,%r5
+	xgr	%r6,%r6
+	xgr	%r7,%r7
+	xgr	%r10,%r10
 	mvc	__PT_R8(64,%r11),__LC_SAVE_AREA_ASYNC
 	stmg	%r8,%r9,__PT_PSW(%r11)
 	lghi	%r1,__LC_EXT_PARAMS2
@@ -1046,6 +1083,16 @@ ENTRY(mcck_int_handler)
 .Lmcck_skip:
 	lghi	%r14,__LC_GPREGS_SAVE_AREA+64
 	stmg	%r0,%r7,__PT_R0(%r11)
+	# clear user controlled registers to prevent speculative use
+	xgr	%r0,%r0
+	xgr	%r1,%r1
+	xgr	%r2,%r2
+	xgr	%r3,%r3
+	xgr	%r4,%r4
+	xgr	%r5,%r5
+	xgr	%r6,%r6
+	xgr	%r7,%r7
+	xgr	%r10,%r10
 	mvc	__PT_R8(64,%r11),0(%r14)
 	stmg	%r8,%r9,__PT_PSW(%r11)
 	xc	__PT_FLAGS(8,%r11),__PT_FLAGS(%r11)
-- 
2.7.4