s390: introduce execute-trampolines for branches

Add CONFIG_EXPOLINE to enable the use of the new -mindirect-branch= and
-mfunction_return= compiler options to create a kernel fortified against
the specte v2 attack.

With CONFIG_EXPOLINE=y all indirect branches will be issued with an
execute type instruction. For z10 or newer the EXRL instruction will
be used, for older machines the EX instruction. The typical indirect
call

	basr	%r14,%r1

is replaced with a PC relative call to a new thunk

	brasl	%r14,__s390x_indirect_jump_r1

The thunk contains the EXRL/EX instruction to the indirect branch

__s390x_indirect_jump_r1:
	exrl	0,0f
	j	.
0:	br	%r1

The detour via the execute type instruction has a performance impact.
To get rid of the detour the new kernel parameter "nospectre_v2" and
"spectre_v2=[on,off,auto]" can be used. If the parameter is specified
the kernel and module code will be patched at runtime.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index d514e25095c2..0636229dae22 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -557,6 +557,34 @@ config KERNEL_NOBP
 
 	  If unsure, say N.
 
+config EXPOLINE
+	def_bool n
+	prompt "Avoid speculative indirect branches in the kernel"
+	help
+	  Compile the kernel with the expoline compiler options to guard
+	  against kernel-to-user data leaks by avoiding speculative indirect
+	  branches.
+	  Requires a compiler with -mindirect-branch=thunk support for full
+	  protection. The kernel may run slower.
+
+	  If unsure, say N.
+
+choice
+	prompt "Expoline default"
+	depends on EXPOLINE
+	default EXPOLINE_FULL
+
+config EXPOLINE_OFF
+	bool "spectre_v2=off"
+
+config EXPOLINE_MEDIUM
+	bool "spectre_v2=auto"
+
+config EXPOLINE_FULL
+	bool "spectre_v2=on"
+
+endchoice
+
 endmenu
 
 menu "Memory setup"