The patch below does not apply to the 4.9-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 23a4d7fd34856da8218c4cfc23dba7a6ec0a423a Mon Sep 17 00:00:00 2001

From: Martin Schwidefsky schwidefsky@de.ibm.com Date: Wed, 25 Apr 2018 18:35:26 +0200 Subject: [PATCH] s390/ftrace: use expoline for indirect branches

The return from the ftrace_stub, _mcount, ftrace_caller and return_to_handler functions is done with "br %r14" and "br %r1". These are indirect branches as well and need to use execute trampolines for CONFIG_EXPOLINE=y.

The ftrace_caller function is a special case as it returns to the start of a function and may only use %r0 and %r1. For a pre z10 machine the standard execute trampoline uses a LARL + EX to do this, but this requires *two* registers in the range %r1..%r15. To get around this the 'br %r1' located in the lowcore is used, then the EX instruction does not need an address register. But the lowcore trick may only be used for pre z14 machines, with noexec=on the mapping for the first page may not contain instructions. The solution for that is an ALTERNATIVE in the expoline THUNK generated by 'GEN_BR_THUNK %r1' to switch to EXRL, this relies on the fact that a machine that supports noexec=on has EXRL as well.

Cc: stable@vger.kernel.org # 4.16 Fixes: f19fbd5ed6 ("s390: introduce execute-trampolines for branches") Signed-off-by: Martin Schwidefsky schwidefsky@de.ibm.com

diff --git a/arch/s390/include/asm/nospec-insn.h b/arch/s390/include/asm/nospec-insn.h index 440689cbcf51..7d7640e1cf90 100644 --- a/arch/s390/include/asm/nospec-insn.h +++ b/arch/s390/include/asm/nospec-insn.h @@ -2,12 +2,16 @@ #ifndef _ASM_S390_NOSPEC_ASM_H #define _ASM_S390_NOSPEC_ASM_H

+#include <asm/alternative-asm.h> +#include <asm/asm-offsets.h> #include <asm/dwarf.h>

#ifdef __ASSEMBLY__

#ifdef CONFIG_EXPOLINE

+_LC_BR_R1 = __LC_BR_R1 + /* * The expoline macros are used to create thunks in the same format * as gcc generates them. The 'comdat' section flag makes sure that @@ -78,13 +82,21 @@ .endm

.macro __THUNK_EX_BR reg,ruse + # Be very careful when adding instructions to this macro! + # The ALTERNATIVE replacement code has a .+10 which targets + # the "br \reg" after the code has been patched. #ifdef CONFIG_HAVE_MARCH_Z10_FEATURES exrl 0,555f j . #else + .ifc \reg,%r1 + ALTERNATIVE "ex %r0,_LC_BR_R1", ".insn ril,0xc60000000000,0,.+10", 35 + j . + .else larl \ruse,555f ex 0,0(\ruse) j . + .endif #endif 555: br \reg .endm diff --git a/arch/s390/kernel/asm-offsets.c b/arch/s390/kernel/asm-offsets.c index eb2a5c0443cd..11aea745a2a6 100644 --- a/arch/s390/kernel/asm-offsets.c +++ b/arch/s390/kernel/asm-offsets.c @@ -181,6 +181,7 @@ int main(void) OFFSET(__LC_MACHINE_FLAGS, lowcore, machine_flags); OFFSET(__LC_PREEMPT_COUNT, lowcore, preempt_count); OFFSET(__LC_GMAP, lowcore, gmap); + OFFSET(__LC_BR_R1, lowcore, br_r1_trampoline); /* software defined ABI-relevant lowcore locations 0xe00 - 0xe20 */ OFFSET(__LC_DUMP_REIPL, lowcore, ipib); /* hardware defined lowcore locations 0x1000 - 0x18ff */ diff --git a/arch/s390/kernel/mcount.S b/arch/s390/kernel/mcount.S index 82df7d80fab2..27110f3294ed 100644 --- a/arch/s390/kernel/mcount.S +++ b/arch/s390/kernel/mcount.S @@ -9,13 +9,17 @@ #include <linux/linkage.h> #include <asm/asm-offsets.h> #include <asm/ftrace.h> +#include <asm/nospec-insn.h> #include <asm/ptrace.h> #include <asm/export.h>

+ GEN_BR_THUNK %r1 + GEN_BR_THUNK %r14 + .section .kprobes.text, "ax"

ENTRY(ftrace_stub) - br %r14 + BR_EX %r14

#define STACK_FRAME_SIZE (STACK_FRAME_OVERHEAD + __PT_SIZE) #define STACK_PTREGS (STACK_FRAME_OVERHEAD) @@ -23,7 +27,7 @@ ENTRY(ftrace_stub) #define STACK_PTREGS_PSW (STACK_PTREGS + __PT_PSW)

ENTRY(_mcount) - br %r14 + BR_EX %r14

EXPORT_SYMBOL(_mcount)

@@ -53,7 +57,7 @@ ENTRY(ftrace_caller) #endif lgr %r3,%r14 la %r5,STACK_PTREGS(%r15) - basr %r14,%r1 + BASR_EX %r14,%r1 #ifdef CONFIG_FUNCTION_GRAPH_TRACER # The j instruction gets runtime patched to a nop instruction. # See ftrace_enable_ftrace_graph_caller. @@ -68,7 +72,7 @@ ftrace_graph_caller_end: #endif lg %r1,(STACK_PTREGS_PSW+8)(%r15) lmg %r2,%r15,(STACK_PTREGS_GPRS+2*8)(%r15) - br %r1 + BR_EX %r1

#ifdef CONFIG_FUNCTION_GRAPH_TRACER

@@ -81,6 +85,6 @@ ENTRY(return_to_handler) aghi %r15,STACK_FRAME_OVERHEAD lgr %r14,%r2 lmg %r2,%r5,32(%r15) - br %r14 + BR_EX %r14

#endif