Patch "s390/kvm: do not rely on the ILC on kvm host protection fauls" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

s390/kvm: do not rely on the ILC on kvm host protection fauls

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
s390-kvm-do-not-rely-on-the-ilc-on-kvm-host-protection-fauls.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Christian Borntraeger <[email protected]>
Date: Mon, 15 May 2017 14:11:03 +0200
Subject: s390/kvm: do not rely on the ILC on kvm host protection fauls

From: Christian Borntraeger <[email protected]>

commit c0e7bb38c07cbd8269549ee0a0566021a3c729de upstream.

For most cases a protection exception in the host (e.g. copy on write or dirty tracking) on the sie instruction will indicate an instruction length of 4. Turns out that there are some corner cases (e.g. runtime instrumentation) where this is not necessarily true and the ILC is unpredictable.

Let's replace our 4 byte rewind_pad with 3 byte nops to prepare for all possible ILCs.

---
 arch/s390/kernel/entry.S |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

@@ -229,12 +229,17 @@ ENTRY(sie64a)
 	lctlg	%c1,%c1,__LC_USER_ASCE		# load primary asce
 .Lsie_done:
 # some program checks are suppressing. C code (e.g. do_protection_exception)
-# will rewind the PSW by the ILC, which is 4 bytes in case of SIE. Other
-# instructions between sie64a and .Lsie_done should not cause program
-# interrupts. So lets use a nop (47 00 00 00) as a landing pad.
+# will rewind the PSW by the ILC, which is often 4 bytes in case of SIE. There
+# are some corner cases (e.g. runtime instrumentation) where ILC is unpredictable.
+# Other instructions between sie64a and .Lsie_done should not cause program
+# interrupts. So lets use 3 nops as a landing pad for all possible rewinds.
 # See also .Lcleanup_sie
-.Lrewind_pad:
-	nop	0
+.Lrewind_pad6:
+	nopr	7
+.Lrewind_pad4:
+	nopr	7
+.Lrewind_pad2:
+	nopr	7
 	.globl sie_exit
 sie_exit:
 	lg	%r14,__SF_EMPTY+8(%r15)		# load guest register save area

@@ -247,7 +252,9 @@ sie_exit:
 	stg	%r14,__SF_EMPTY+16(%r15)	# set exit reason code
 	j	sie_exit
 
-	EX_TABLE(.Lrewind_pad,.Lsie_fault)
+	EX_TABLE(.Lrewind_pad6,.Lsie_fault)
+	EX_TABLE(.Lrewind_pad4,.Lsie_fault)
+	EX_TABLE(.Lrewind_pad2,.Lsie_fault)
 	EX_TABLE(sie_exit,.Lsie_fault)
 #endif
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/s390-kvm-do-not-rely-on-the-ilc-on-kvm-host-protection-fauls.patch