[PATCH 5.15 102/247] s390/mcck: isolate SIE instruction when setting CIF_MCCK_GUEST flag

[Greg Kroah-Hartman]

From: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>

[ Upstream commit 29ccaa4b35ea874ddd50518e5c2c746b9238a792 ]

Commit d768bd892fc8 ("s390: add options to change branch prediction
behaviour for the kernel") introduced .Lsie_exit label - supposedly
to fence off SIE instruction. However, the corresponding address
range length .Lsie_crit_mcck_length was not updated, which led to
BPON code potentionally marked with CIF_MCCK_GUEST flag.

Both .Lsie_exit and .Lsie_crit_mcck_length were removed with commit
0b0ed657fe00 ("s390: remove critical section cleanup from entry.S"),
but the issue persisted - currently BPOFF and BPENTER macros might
get wrongly considered by the machine check handler as a guest.

Fixes: d768bd892fc8 ("s390: add options to change branch prediction behaviour for the kernel")
Reviewed-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/s390/kernel/entry.S | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 4c9b967290ae..d530eb4dc413 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -248,6 +248,10 @@ ENTRY(sie64a)
 	BPEXIT	__SF_SIE_FLAGS(%r15),(_TIF_ISOLATE_BP|_TIF_ISOLATE_BP_GUEST)
 .Lsie_entry:
 	sie	0(%r14)
+# Let the next instruction be NOP to avoid triggering a machine check
+# and handling it in a guest as result of the instruction execution.
+	nopr	7
+.Lsie_leave:
 	BPOFF
 	BPENTER	__SF_SIE_FLAGS(%r15),(_TIF_ISOLATE_BP|_TIF_ISOLATE_BP_GUEST)
 .Lsie_skip:
@@ -536,7 +540,7 @@ ENTRY(mcck_int_handler)
 	jno	.Lmcck_panic
 #if IS_ENABLED(CONFIG_KVM)
 	OUTSIDE	%r9,.Lsie_gmap,.Lsie_done,6f
-	OUTSIDE	%r9,.Lsie_entry,.Lsie_skip,4f
+	OUTSIDE	%r9,.Lsie_entry,.Lsie_leave,4f
 	oi	__LC_CPU_FLAGS+7, _CIF_MCCK_GUEST
 	j	5f
 4:	CHKSTG	.Lmcck_panic
-- 
2.35.1