[PATCH 4.14 50/80] s390: introduce execute-trampolines for branches

From: Greg Kroah-Hartman
Date: Fri Apr 27 2018 - 10:47:10 EST

4.14-stable review patch. If anyone has any objections, please let me know.

------------------

From: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>


[ Upstream commit f19fbd5ed642dc31c809596412dab1ed56f2f156 ]

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@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/s390/Kconfig | 28 ++++++++
arch/s390/Makefile | 10 +++
arch/s390/include/asm/lowcore.h | 4 -
arch/s390/include/asm/nospec-branch.h | 18 +++++
arch/s390/kernel/Makefile | 4 +
arch/s390/kernel/entry.S | 113 ++++++++++++++++++++++++++--------
arch/s390/kernel/module.c | 62 +++++++++++++++---
arch/s390/kernel/nospec-branch.c | 101 ++++++++++++++++++++++++++++++
arch/s390/kernel/setup.c | 4 +
arch/s390/kernel/smp.c | 1
arch/s390/kernel/vmlinux.lds.S | 14 ++++
drivers/s390/char/Makefile | 2
12 files changed, 326 insertions(+), 35 deletions(-)
create mode 100644 arch/s390/include/asm/nospec-branch.h
create mode 100644 arch/s390/kernel/nospec-branch.c

--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -555,6 +555,34 @@ 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"
--- a/arch/s390/Makefile
+++ b/arch/s390/Makefile
@@ -81,6 +81,16 @@ ifeq ($(call cc-option-yn,-mwarn-dynamic
cflags-$(CONFIG_WARN_DYNAMIC_STACK) += -mwarn-dynamicstack
endif

+ifdef CONFIG_EXPOLINE
+ ifeq ($(call cc-option-yn,$(CC_FLAGS_MARCH) -mindirect-branch=thunk),y)
+ CC_FLAGS_EXPOLINE := -mindirect-branch=thunk
+ CC_FLAGS_EXPOLINE += -mfunction-return=thunk
+ CC_FLAGS_EXPOLINE += -mindirect-branch-table
+ export CC_FLAGS_EXPOLINE
+ cflags-y += $(CC_FLAGS_EXPOLINE)
+ endif
+endif
+
ifdef CONFIG_FUNCTION_TRACER
# make use of hotpatch feature if the compiler supports it
cc_hotpatch := -mhotpatch=0,3
--- a/arch/s390/include/asm/lowcore.h
+++ b/arch/s390/include/asm/lowcore.h
@@ -140,7 +140,9 @@ struct lowcore {
/* Per cpu primary space access list */
__u32 paste[16]; /* 0x0400 */

- __u8 pad_0x04c0[0x0e00-0x0440]; /* 0x0440 */
+ /* br %r1 trampoline */
+ __u16 br_r1_trampoline; /* 0x0440 */
+ __u8 pad_0x0442[0x0e00-0x0442]; /* 0x0442 */

/*
* 0xe00 contains the address of the IPL Parameter Information
--- /dev/null
+++ b/arch/s390/include/asm/nospec-branch.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_S390_EXPOLINE_H
+#define _ASM_S390_EXPOLINE_H
+
+#ifndef __ASSEMBLY__
+
+#include <linux/types.h>
+
+extern int nospec_call_disable;
+extern int nospec_return_disable;
+
+void nospec_init_branches(void);
+void nospec_call_revert(s32 *start, s32 *end);
+void nospec_return_revert(s32 *start, s32 *end);
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_S390_EXPOLINE_H */
--- a/arch/s390/kernel/Makefile
+++ b/arch/s390/kernel/Makefile
@@ -29,6 +29,7 @@ UBSAN_SANITIZE_early.o := n
#
ifneq ($(CC_FLAGS_MARCH),-march=z900)
CFLAGS_REMOVE_als.o += $(CC_FLAGS_MARCH)
+CFLAGS_REMOVE_als.o += $(CC_FLAGS_EXPOLINE)
CFLAGS_als.o += -march=z900
AFLAGS_REMOVE_head.o += $(CC_FLAGS_MARCH)
AFLAGS_head.o += -march=z900
@@ -61,6 +62,9 @@ obj-y += entry.o reipl.o relocate_kernel

extra-y += head.o head64.o vmlinux.lds

+obj-$(CONFIG_EXPOLINE) += nospec-branch.o
+CFLAGS_REMOVE_expoline.o += $(CC_FLAGS_EXPOLINE)
+
obj-$(CONFIG_MODULES) += module.o
obj-$(CONFIG_SMP) += smp.o
obj-$(CONFIG_SCHED_TOPOLOGY) += topology.o
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -221,6 +221,68 @@ _PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCAL
.popsection
.endm

+#ifdef CONFIG_EXPOLINE
+
+ .macro GEN_BR_THUNK name,reg,tmp
+ .section .text.\name,"axG",@progbits,\name,comdat
+ .globl \name
+ .hidden \name
+ .type \name,@function
+\name:
+ .cfi_startproc
+#ifdef CONFIG_HAVE_MARCH_Z10_FEATURES
+ exrl 0,0f
+#else
+ larl \tmp,0f
+ ex 0,0(\tmp)
+#endif
+ j .
+0: br \reg
+ .cfi_endproc
+ .endm
+
+ GEN_BR_THUNK __s390x_indirect_jump_r1use_r9,%r9,%r1
+ GEN_BR_THUNK __s390x_indirect_jump_r1use_r14,%r14,%r1
+ GEN_BR_THUNK __s390x_indirect_jump_r11use_r14,%r14,%r11
+
+ .macro BASR_R14_R9
+0: brasl %r14,__s390x_indirect_jump_r1use_r9
+ .pushsection .s390_indirect_branches,"a",@progbits
+ .long 0b-.
+ .popsection
+ .endm
+
+ .macro BR_R1USE_R14
+0: jg __s390x_indirect_jump_r1use_r14
+ .pushsection .s390_indirect_branches,"a",@progbits
+ .long 0b-.
+ .popsection
+ .endm
+
+ .macro BR_R11USE_R14
+0: jg __s390x_indirect_jump_r11use_r14
+ .pushsection .s390_indirect_branches,"a",@progbits
+ .long 0b-.
+ .popsection
+ .endm
+
+#else /* CONFIG_EXPOLINE */
+
+ .macro BASR_R14_R9
+ basr %r14,%r9
+ .endm
+
+ .macro BR_R1USE_R14
+ br %r14
+ .endm
+
+ .macro BR_R11USE_R14
+ br %r14
+ .endm
+
+#endif /* CONFIG_EXPOLINE */
+
+
.section .kprobes.text, "ax"
.Ldummy:
/*
@@ -236,7 +298,7 @@ _PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCAL
ENTRY(__bpon)
.globl __bpon
BPON
- br %r14
+ BR_R1USE_R14

/*
* Scheduler resume function, called by switch_to
@@ -261,9 +323,9 @@ ENTRY(__switch_to)
mvc __LC_CURRENT_PID(4,%r0),__TASK_pid(%r3) # store pid of next
lmg %r6,%r15,__SF_GPRS(%r15) # load gprs of next task
TSTMSK __LC_MACHINE_FLAGS,MACHINE_FLAG_LPP
- bzr %r14
+ jz 0f
.insn s,0xb2800000,__LC_LPP # set program parameter
- br %r14
+0: BR_R1USE_R14

.L__critical_start:

@@ -330,7 +392,7 @@ sie_exit:
xgr %r5,%r5
lmg %r6,%r14,__SF_GPRS(%r15) # restore kernel registers
lg %r2,__SF_EMPTY+16(%r15) # return exit reason code
- br %r14
+ BR_R1USE_R14
.Lsie_fault:
lghi %r14,-EFAULT
stg %r14,__SF_EMPTY+16(%r15) # set exit reason code
@@ -389,7 +451,7 @@ ENTRY(system_call)
lgf %r9,0(%r8,%r10) # get system call add.
TSTMSK __TI_flags(%r12),_TIF_TRACE
jnz .Lsysc_tracesys
- basr %r14,%r9 # call sys_xxxx
+ BASR_R14_R9 # call sys_xxxx
stg %r2,__PT_R2(%r11) # store return value

.Lsysc_return:
@@ -566,7 +628,7 @@ ENTRY(system_call)
lmg %r3,%r7,__PT_R3(%r11)
stg %r7,STACK_FRAME_OVERHEAD(%r15)
lg %r2,__PT_ORIG_GPR2(%r11)
- basr %r14,%r9 # call sys_xxx
+ BASR_R14_R9 # call sys_xxx
stg %r2,__PT_R2(%r11) # store return value
.Lsysc_tracenogo:
TSTMSK __TI_flags(%r12),_TIF_TRACE
@@ -590,7 +652,7 @@ ENTRY(ret_from_fork)
lmg %r9,%r10,__PT_R9(%r11) # load gprs
ENTRY(kernel_thread_starter)
la %r2,0(%r10)
- basr %r14,%r9
+ BASR_R14_R9
j .Lsysc_tracenogo

/*
@@ -667,9 +729,9 @@ ENTRY(pgm_check_handler)
nill %r10,0x007f
sll %r10,2
je .Lpgm_return
- lgf %r1,0(%r10,%r1) # load address of handler routine
+ lgf %r9,0(%r10,%r1) # load address of handler routine
lgr %r2,%r11 # pass pointer to pt_regs
- basr %r14,%r1 # branch to interrupt-handler
+ BASR_R14_R9 # branch to interrupt-handler
.Lpgm_return:
LOCKDEP_SYS_EXIT
tm __PT_PSW+1(%r11),0x01 # returning to user ?
@@ -979,7 +1041,7 @@ ENTRY(psw_idle)
stpt __TIMER_IDLE_ENTER(%r2)
.Lpsw_idle_lpsw:
lpswe __SF_EMPTY(%r15)
- br %r14
+ BR_R1USE_R14
.Lpsw_idle_end:

/*
@@ -993,7 +1055,7 @@ ENTRY(save_fpu_regs)
lg %r2,__LC_CURRENT
aghi %r2,__TASK_thread
TSTMSK __LC_CPU_FLAGS,_CIF_FPU
- bor %r14
+ jo .Lsave_fpu_regs_exit
stfpc __THREAD_FPU_fpc(%r2)
lg %r3,__THREAD_FPU_regs(%r2)
TSTMSK __LC_MACHINE_FLAGS,MACHINE_FLAG_VX
@@ -1020,7 +1082,8 @@ ENTRY(save_fpu_regs)
std 15,120(%r3)
.Lsave_fpu_regs_done:
oi __LC_CPU_FLAGS+7,_CIF_FPU
- br %r14
+.Lsave_fpu_regs_exit:
+ BR_R1USE_R14
.Lsave_fpu_regs_end:
EXPORT_SYMBOL(save_fpu_regs)

@@ -1038,7 +1101,7 @@ load_fpu_regs:
lg %r4,__LC_CURRENT
aghi %r4,__TASK_thread
TSTMSK __LC_CPU_FLAGS,_CIF_FPU
- bnor %r14
+ jno .Lload_fpu_regs_exit
lfpc __THREAD_FPU_fpc(%r4)
TSTMSK __LC_MACHINE_FLAGS,MACHINE_FLAG_VX
lg %r4,__THREAD_FPU_regs(%r4) # %r4 <- reg save area
@@ -1065,7 +1128,8 @@ load_fpu_regs:
ld 15,120(%r4)
.Lload_fpu_regs_done:
ni __LC_CPU_FLAGS+7,255-_CIF_FPU
- br %r14
+.Lload_fpu_regs_exit:
+ BR_R1USE_R14
.Lload_fpu_regs_end:

.L__critical_end:
@@ -1237,7 +1301,7 @@ cleanup_critical:
jl 0f
clg %r9,BASED(.Lcleanup_table+104) # .Lload_fpu_regs_end
jl .Lcleanup_load_fpu_regs
-0: br %r14
+0: BR_R11USE_R14

.align 8
.Lcleanup_table:
@@ -1273,7 +1337,7 @@ cleanup_critical:
ni __SIE_PROG0C+3(%r9),0xfe # no longer in SIE
lctlg %c1,%c1,__LC_USER_ASCE # load primary asce
larl %r9,sie_exit # skip forward to sie_exit
- br %r14
+ BR_R11USE_R14
#endif

.Lcleanup_system_call:
@@ -1326,7 +1390,7 @@ cleanup_critical:
stg %r15,56(%r11) # r15 stack pointer
# set new psw address and exit
larl %r9,.Lsysc_do_svc
- br %r14
+ BR_R11USE_R14
.Lcleanup_system_call_insn:
.quad system_call
.quad .Lsysc_stmg
@@ -1338,7 +1402,7 @@ cleanup_critical:

.Lcleanup_sysc_tif:
larl %r9,.Lsysc_tif
- br %r14
+ BR_R11USE_R14

.Lcleanup_sysc_restore:
# check if stpt has been executed
@@ -1355,14 +1419,14 @@ cleanup_critical:
mvc 0(64,%r11),__PT_R8(%r9)
lmg %r0,%r7,__PT_R0(%r9)
1: lmg %r8,%r9,__LC_RETURN_PSW
- br %r14
+ BR_R11USE_R14
.Lcleanup_sysc_restore_insn:
.quad .Lsysc_exit_timer
.quad .Lsysc_done - 4

.Lcleanup_io_tif:
larl %r9,.Lio_tif
- br %r14
+ BR_R11USE_R14

.Lcleanup_io_restore:
# check if stpt has been executed
@@ -1376,7 +1440,7 @@ cleanup_critical:
mvc 0(64,%r11),__PT_R8(%r9)
lmg %r0,%r7,__PT_R0(%r9)
1: lmg %r8,%r9,__LC_RETURN_PSW
- br %r14
+ BR_R11USE_R14
.Lcleanup_io_restore_insn:
.quad .Lio_exit_timer
.quad .Lio_done - 4
@@ -1429,17 +1493,17 @@ cleanup_critical:
# prepare return psw
nihh %r8,0xfcfd # clear irq & wait state bits
lg %r9,48(%r11) # return from psw_idle
- br %r14
+ BR_R11USE_R14
.Lcleanup_idle_insn:
.quad .Lpsw_idle_lpsw

.Lcleanup_save_fpu_regs:
larl %r9,save_fpu_regs
- br %r14
+ BR_R11USE_R14

.Lcleanup_load_fpu_regs:
larl %r9,load_fpu_regs
- br %r14
+ BR_R11USE_R14

/*
* Integer constants
@@ -1459,7 +1523,6 @@ cleanup_critical:
.Lsie_crit_mcck_length:
.quad .Lsie_skip - .Lsie_entry
#endif
-
.section .rodata, "a"
#define SYSCALL(esame,emu) .long esame
.globl sys_call_table
--- a/arch/s390/kernel/module.c
+++ b/arch/s390/kernel/module.c
@@ -32,6 +32,8 @@
#include <linux/moduleloader.h>
#include <linux/bug.h>
#include <asm/alternative.h>
+#include <asm/nospec-branch.h>
+#include <asm/facility.h>

#if 0
#define DEBUGP printk
@@ -169,7 +171,11 @@ int module_frob_arch_sections(Elf_Ehdr *
me->arch.got_offset = me->core_layout.size;
me->core_layout.size += me->arch.got_size;
me->arch.plt_offset = me->core_layout.size;
- me->core_layout.size += me->arch.plt_size;
+ if (me->arch.plt_size) {
+ if (IS_ENABLED(CONFIG_EXPOLINE) && !nospec_call_disable)
+ me->arch.plt_size += PLT_ENTRY_SIZE;
+ me->core_layout.size += me->arch.plt_size;
+ }
return 0;
}

@@ -323,9 +329,21 @@ static int apply_rela(Elf_Rela *rela, El
unsigned int *ip;
ip = me->core_layout.base + me->arch.plt_offset +
info->plt_offset;
- ip[0] = 0x0d10e310; /* basr 1,0; lg 1,10(1); br 1 */
- ip[1] = 0x100a0004;
- ip[2] = 0x07f10000;
+ ip[0] = 0x0d10e310; /* basr 1,0 */
+ ip[1] = 0x100a0004; /* lg 1,10(1) */
+ if (IS_ENABLED(CONFIG_EXPOLINE) &&
+ !nospec_call_disable) {
+ unsigned int *ij;
+ ij = me->core_layout.base +
+ me->arch.plt_offset +
+ me->arch.plt_size - PLT_ENTRY_SIZE;
+ ip[2] = 0xa7f40000 + /* j __jump_r1 */
+ (unsigned int)(u16)
+ (((unsigned long) ij - 8 -
+ (unsigned long) ip) / 2);
+ } else {
+ ip[2] = 0x07f10000; /* br %r1 */
+ }
ip[3] = (unsigned int) (val >> 32);
ip[4] = (unsigned int) val;
info->plt_initialized = 1;
@@ -431,16 +449,42 @@ int module_finalize(const Elf_Ehdr *hdr,
struct module *me)
{
const Elf_Shdr *s;
- char *secstrings;
+ char *secstrings, *secname;
+ void *aseg;
+
+ if (IS_ENABLED(CONFIG_EXPOLINE) &&
+ !nospec_call_disable && me->arch.plt_size) {
+ unsigned int *ij;
+
+ ij = me->core_layout.base + me->arch.plt_offset +
+ me->arch.plt_size - PLT_ENTRY_SIZE;
+ if (test_facility(35)) {
+ ij[0] = 0xc6000000; /* exrl %r0,.+10 */
+ ij[1] = 0x0005a7f4; /* j . */
+ ij[2] = 0x000007f1; /* br %r1 */
+ } else {
+ ij[0] = 0x44000000 | (unsigned int)
+ offsetof(struct lowcore, br_r1_trampoline);
+ ij[1] = 0xa7f40000; /* j . */
+ }
+ }

secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
- if (!strcmp(".altinstructions", secstrings + s->sh_name)) {
- /* patch .altinstructions */
- void *aseg = (void *)s->sh_addr;
+ aseg = (void *) s->sh_addr;
+ secname = secstrings + s->sh_name;

+ if (!strcmp(".altinstructions", secname))
+ /* patch .altinstructions */
apply_alternatives(aseg, aseg + s->sh_size);
- }
+
+ if (IS_ENABLED(CONFIG_EXPOLINE) &&
+ (!strcmp(".nospec_call_table", secname)))
+ nospec_call_revert(aseg, aseg + s->sh_size);
+
+ if (IS_ENABLED(CONFIG_EXPOLINE) &&
+ (!strcmp(".nospec_return_table", secname)))
+ nospec_return_revert(aseg, aseg + s->sh_size);
}

jump_label_apply_nops(me);
--- /dev/null
+++ b/arch/s390/kernel/nospec-branch.c
@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/module.h>
+#include <asm/facility.h>
+#include <asm/nospec-branch.h>
+
+int nospec_call_disable = IS_ENABLED(EXPOLINE_OFF);
+int nospec_return_disable = !IS_ENABLED(EXPOLINE_FULL);
+
+static int __init nospectre_v2_setup_early(char *str)
+{
+ nospec_call_disable = 1;
+ nospec_return_disable = 1;
+ return 0;
+}
+early_param("nospectre_v2", nospectre_v2_setup_early);
+
+static int __init spectre_v2_setup_early(char *str)
+{
+ if (str && !strncmp(str, "on", 2)) {
+ nospec_call_disable = 0;
+ nospec_return_disable = 0;
+ }
+ if (str && !strncmp(str, "off", 3)) {
+ nospec_call_disable = 1;
+ nospec_return_disable = 1;
+ }
+ if (str && !strncmp(str, "auto", 4)) {
+ nospec_call_disable = 0;
+ nospec_return_disable = 1;
+ }
+ return 0;
+}
+early_param("spectre_v2", spectre_v2_setup_early);
+
+static void __init_or_module __nospec_revert(s32 *start, s32 *end)
+{
+ enum { BRCL_EXPOLINE, BRASL_EXPOLINE } type;
+ u8 *instr, *thunk, *br;
+ u8 insnbuf[6];
+ s32 *epo;
+
+ /* Second part of the instruction replace is always a nop */
+ memcpy(insnbuf + 2, (char[]) { 0x47, 0x00, 0x00, 0x00 }, 4);
+ for (epo = start; epo < end; epo++) {
+ instr = (u8 *) epo + *epo;
+ if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x04)
+ type = BRCL_EXPOLINE; /* brcl instruction */
+ else if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x05)
+ type = BRASL_EXPOLINE; /* brasl instruction */
+ else
+ continue;
+ thunk = instr + (*(int *)(instr + 2)) * 2;
+ if (thunk[0] == 0xc6 && thunk[1] == 0x00)
+ /* exrl %r0,<target-br> */
+ br = thunk + (*(int *)(thunk + 2)) * 2;
+ else if (thunk[0] == 0xc0 && (thunk[1] & 0x0f) == 0x00 &&
+ thunk[6] == 0x44 && thunk[7] == 0x00 &&
+ (thunk[8] & 0x0f) == 0x00 && thunk[9] == 0x00 &&
+ (thunk[1] & 0xf0) == (thunk[8] & 0xf0))
+ /* larl %rx,<target br> + ex %r0,0(%rx) */
+ br = thunk + (*(int *)(thunk + 2)) * 2;
+ else
+ continue;
+ if (br[0] != 0x07 || (br[1] & 0xf0) != 0xf0)
+ continue;
+ switch (type) {
+ case BRCL_EXPOLINE:
+ /* brcl to thunk, replace with br + nop */
+ insnbuf[0] = br[0];
+ insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
+ break;
+ case BRASL_EXPOLINE:
+ /* brasl to thunk, replace with basr + nop */
+ insnbuf[0] = 0x0d;
+ insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
+ break;
+ }
+
+ s390_kernel_write(instr, insnbuf, 6);
+ }
+}
+
+void __init_or_module nospec_call_revert(s32 *start, s32 *end)
+{
+ if (nospec_call_disable)
+ __nospec_revert(start, end);
+}
+
+void __init_or_module nospec_return_revert(s32 *start, s32 *end)
+{
+ if (nospec_return_disable)
+ __nospec_revert(start, end);
+}
+
+extern s32 __nospec_call_start[], __nospec_call_end[];
+extern s32 __nospec_return_start[], __nospec_return_end[];
+void __init nospec_init_branches(void)
+{
+ nospec_call_revert(__nospec_call_start, __nospec_call_end);
+ nospec_return_revert(__nospec_return_start, __nospec_return_end);
+}
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -67,6 +67,7 @@
#include <asm/sysinfo.h>
#include <asm/numa.h>
#include <asm/alternative.h>
+#include <asm/nospec-branch.h>
#include "entry.h"

/*
@@ -384,6 +385,7 @@ static void __init setup_lowcore(void)
#ifdef CONFIG_SMP
lc->spinlock_lockval = arch_spin_lockval(0);
#endif
+ lc->br_r1_trampoline = 0x07f1; /* br %r1 */

set_prefix((u32)(unsigned long) lc);
lowcore_ptr[0] = lc;
@@ -959,6 +961,8 @@ void __init setup_arch(char **cmdline_p)
set_preferred_console();

apply_alternative_instructions();
+ if (IS_ENABLED(CONFIG_EXPOLINE))
+ nospec_init_branches();

/* Setup zfcpdump support */
setup_zfcpdump();
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -228,6 +228,7 @@ static int pcpu_alloc_lowcore(struct pcp
lc->mcesad = mcesa_origin | mcesa_bits;
lc->cpu_nr = cpu;
lc->spinlock_lockval = arch_spin_lockval(cpu);
+ lc->br_r1_trampoline = 0x07f1; /* br %r1 */
if (vdso_alloc_per_cpu(lc))
goto out;
lowcore_ptr[cpu] = lc;
--- a/arch/s390/kernel/vmlinux.lds.S
+++ b/arch/s390/kernel/vmlinux.lds.S
@@ -128,6 +128,20 @@ SECTIONS
*(.altinstr_replacement)
}

+ /*
+ * Table with the patch locations to undo expolines
+ */
+ .nospec_call_table : {
+ __nospec_call_start = . ;
+ *(.s390_indirect*)
+ __nospec_call_end = . ;
+ }
+ .nospec_return_table : {
+ __nospec_return_start = . ;
+ *(.s390_return*)
+ __nospec_return_end = . ;
+ }
+
/* early.c uses stsi, which requires page aligned data. */
. = ALIGN(PAGE_SIZE);
INIT_DATA_SECTION(0x100)
--- a/drivers/s390/char/Makefile
+++ b/drivers/s390/char/Makefile
@@ -17,6 +17,8 @@ CFLAGS_REMOVE_sclp_early_core.o += $(CC_
CFLAGS_sclp_early_core.o += -march=z900
endif

+CFLAGS_REMOVE_sclp_early_core.o += $(CC_FLAGS_EXPOLINE)
+
obj-y += ctrlchar.o keyboard.o defkeymap.o sclp.o sclp_rw.o sclp_quiesce.o \
sclp_cmd.o sclp_config.o sclp_cpi_sys.o sclp_ocf.o sclp_ctl.o \
sclp_early.o sclp_early_core.o