Patch "s390/noexec: execute kexec datamover without DAT" has been added to the 4.14-stable tree

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

s390/noexec: execute kexec datamover without DAT

to the 4.14-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-noexec-execute-kexec-datamover-without-dat.patch
and it can be found in the queue-4.14 subdirectory.

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

From: Heiko Carstens <[email protected]>
Date: Thu, 9 Nov 2017 23:00:14 +0100
Subject: s390/noexec: execute kexec datamover without DAT

From: Heiko Carstens <[email protected]>

commit d0e810eeb3d326978f248b8f0233a2f30f58c72d upstream.

Rebooting into a new kernel with kexec fails (system dies) if tried on a machine that has no-execute support. Reason for this is that the so called datamover code gets executed with DAT on (MMU is active) and the page that contains the datamover is marked as non-executable. Therefore when branching into the datamover an unexpected program check happens and afterwards the machine is dead.

This can be simply avoided by disabling DAT, which also disables any no-execute checks, just before the datamover gets executed.

In fact the first thing done by the datamover is to disable DAT. The code in the datamover that disables DAT can be removed as well.

Thanks to Michael Holzheu and Gerald Schaefer for tracking this down.

---
 arch/s390/kernel/machine_kexec.c   |    1 +
 arch/s390/kernel/relocate_kernel.S |    3 ---
 2 files changed, 1 insertion(+), 3 deletions(-)

@@ -269,6 +269,7 @@ static void __do_machine_kexec(void *dat
 	s390_reset_system();
 	data_mover = (relocate_kernel_t) page_to_phys(image->control_code_page);
 
+	__arch_local_irq_stnsm(0xfb); /* disable DAT - avoid no-execute */
 	/* Call the moving routine */
 	(*data_mover)(&image->head, image->start);
 

@@ -29,7 +29,6 @@
 ENTRY(relocate_kernel)
 		basr	%r13,0		# base address
 	.base:
-		stnsm	sys_msk-.base(%r13),0xfb	# disable DAT
 		stctg	%c0,%c15,ctlregs-.base(%r13)
 		stmg	%r0,%r15,gprregs-.base(%r13)
 		lghi	%r0,3

@@ -103,8 +102,6 @@ ENTRY(relocate_kernel)
 		.align	8
 	load_psw:
 		.long	0x00080000,0x80000000
-	sys_msk:
-		.quad	0
 	ctlregs:
 		.rept	16
 		.quad	0

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

queue-4.14/s390-guarded-storage-fix-possible-memory-corruption.patch queue-4.14/s390-disassembler-add-missing-end-marker-for-e7-table.patch queue-4.14/s390-fix-transactional-execution-control-register-handling.patch queue-4.14/s390-runtime-instrumention-fix-possible-memory-corruption.patch queue-4.14/s390-noexec-execute-kexec-datamover-without-dat.patch