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Subject[PATCH 4.4 268/268] s390/ftrace: use expoline for indirect branches
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

commit 23a4d7fd34856da8218c4cfc23dba7a6ec0a423a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/s390/include/asm/nospec-insn.h | 13 +++++++++++++
arch/s390/kernel/asm-offsets.c | 1 +
arch/s390/kernel/mcount.S | 14 +++++++++-----
3 files changed, 23 insertions(+), 5 deletions(-)

--- a/arch/s390/include/asm/nospec-insn.h
+++ b/arch/s390/include/asm/nospec-insn.h
@@ -2,10 +2,15 @@
#ifndef _ASM_S390_NOSPEC_ASM_H
#define _ASM_S390_NOSPEC_ASM_H

+#include <asm/alternative-asm.h>
+#include <asm/asm-offsets.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
@@ -101,13 +106,21 @@
.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
--- a/arch/s390/kernel/asm-offsets.c
+++ b/arch/s390/kernel/asm-offsets.c
@@ -170,6 +170,7 @@ int main(void)
OFFSET(__LC_MACHINE_FLAGS, _lowcore, machine_flags);
OFFSET(__LC_GMAP, _lowcore, gmap);
OFFSET(__LC_PASTE, _lowcore, paste);
+ 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 */
--- a/arch/s390/kernel/mcount.S
+++ b/arch/s390/kernel/mcount.S
@@ -8,12 +8,16 @@
#include <linux/linkage.h>
#include <asm/asm-offsets.h>
#include <asm/ftrace.h>
+#include <asm/nospec-insn.h>
#include <asm/ptrace.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)
@@ -21,7 +25,7 @@ ENTRY(ftrace_stub)
#define STACK_PTREGS_PSW (STACK_PTREGS + __PT_PSW)

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

ENTRY(ftrace_caller)
.globl ftrace_regs_caller
@@ -49,7 +53,7 @@ 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.
@@ -64,7 +68,7 @@ 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

@@ -77,6 +81,6 @@ ENTRY(return_to_handler)
aghi %r15,STACK_FRAME_OVERHEAD
lgr %r14,%r2
lmg %r2,%r5,32(%r15)
- br %r14
+ BR_EX %r14

#endif

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