[SRU][G][PATCH 1/1] s390: fix fpu restore in entry.S

[frank.heimes at canonical.com]

From: Sven Schnelle <svens at linux.ibm.com>

BugLink: https://bugs.launchpad.net/bugs/1906255

We need to disable interrupts in load_fpu_regs(). Otherwise an
interrupt might come in after the registers are loaded, but before
CIF_FPU is cleared in load_fpu_regs(). When the interrupt returns,
CIF_FPU will be cleared and the registers will never be restored.

The entry.S code usually saves the interrupt state in __SF_EMPTY on the
stack when disabling/restoring interrupts. sie64a however saves the pointer
to the sie control block in __SF_SIE_CONTROL, which references the same
location.  This is non-obvious to the reader. To avoid thrashing the sie
control block pointer in load_fpu_regs(), move the __SIE_* offsets eight
bytes after __SF_EMPTY on the stack.

Cc: <stable at vger.kernel.org> # 5.8
Fixes: 0b0ed657fe00 ("s390: remove critical section cleanup from entry.S")
Reported-by: Pierre Morel <pmorel at linux.ibm.com>
Signed-off-by: Sven Schnelle <svens at linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger at de.ibm.com>
Reviewed-by: Heiko Carstens <hca at linux.ibm.com>
Signed-off-by: Heiko Carstens <hca at linux.ibm.com>
(cherry picked from commit 1179f170b6f0af7bb0b3b7628136eaac450ddf31 linux-next)
Signed-off-by: Frank Heimes <frank.heimes at canonical.com>
---
 arch/s390/kernel/asm-offsets.c | 10 +++++-----
 arch/s390/kernel/entry.S       |  2 ++
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/s390/kernel/asm-offsets.c b/arch/s390/kernel/asm-offsets.c
index 5d8cc1864566..62a18dee4c36 100644
--- a/arch/s390/kernel/asm-offsets.c
+++ b/arch/s390/kernel/asm-offsets.c
@@ -53,11 +53,11 @@ int main(void)
 	/* stack_frame offsets */
 	OFFSET(__SF_BACKCHAIN, stack_frame, back_chain);
 	OFFSET(__SF_GPRS, stack_frame, gprs);
-	OFFSET(__SF_EMPTY, stack_frame, empty1);
-	OFFSET(__SF_SIE_CONTROL, stack_frame, empty1[0]);
-	OFFSET(__SF_SIE_SAVEAREA, stack_frame, empty1[1]);
-	OFFSET(__SF_SIE_REASON, stack_frame, empty1[2]);
-	OFFSET(__SF_SIE_FLAGS, stack_frame, empty1[3]);
+	OFFSET(__SF_EMPTY, stack_frame, empty1[0]);
+	OFFSET(__SF_SIE_CONTROL, stack_frame, empty1[1]);
+	OFFSET(__SF_SIE_SAVEAREA, stack_frame, empty1[2]);
+	OFFSET(__SF_SIE_REASON, stack_frame, empty1[3]);
+	OFFSET(__SF_SIE_FLAGS, stack_frame, empty1[4]);
 	BLANK();
 	/* timeval/timezone offsets for use by vdso */
 	OFFSET(__VDSO_UPD_COUNT, vdso_data, tb_update_count);
diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 969b35b177dd..f314a638fa8d 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -1070,6 +1070,7 @@ EXPORT_SYMBOL(save_fpu_regs)
  *	%r4
  */
 load_fpu_regs:
+	stnsm	__SF_EMPTY(%r15),0xfc
 	lg	%r4,__LC_CURRENT
 	aghi	%r4,__TASK_thread
 	TSTMSK	__LC_CPU_FLAGS,_CIF_FPU
@@ -1101,6 +1102,7 @@ load_fpu_regs:
 .Lload_fpu_regs_done:
 	ni	__LC_CPU_FLAGS+7,255-_CIF_FPU
 .Lload_fpu_regs_exit:
+	ssm	__SF_EMPTY(%r15)
 	BR_EX	%r14
 .Lload_fpu_regs_end:
 ENDPROC(load_fpu_regs)
-- 
2.25.1