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Subject[PATCH 4.4 053/266] powerpc/64s: Add support for software count cache flush
Date
From: Michael Ellerman <mpe@ellerman.id.au>

commit ee13cb249fabdff8b90aaff61add347749280087 upstream.

Some CPU revisions support a mode where the count cache needs to be
flushed by software on context switch. Additionally some revisions may
have a hardware accelerated flush, in which case the software flush
sequence can be shortened.

If we detect the appropriate flag from firmware we patch a branch
into _switch() which takes us to a count cache flush sequence.

That sequence in turn may be patched to return early if we detect that
the CPU supports accelerating the flush sequence in hardware.

Add debugfs support for reporting the state of the flush, as well as
runtime disabling it.

And modify the spectre_v2 sysfs file to report the state of the
software flush.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/powerpc/include/asm/asm-prototypes.h | 21 +++++
arch/powerpc/include/asm/security_features.h | 1
arch/powerpc/kernel/entry_64.S | 54 ++++++++++++++
arch/powerpc/kernel/security.c | 98 +++++++++++++++++++++++++--
4 files changed, 169 insertions(+), 5 deletions(-)
create mode 100644 arch/powerpc/include/asm/asm-prototypes.h

--- /dev/null
+++ b/arch/powerpc/include/asm/asm-prototypes.h
@@ -0,0 +1,21 @@
+#ifndef _ASM_POWERPC_ASM_PROTOTYPES_H
+#define _ASM_POWERPC_ASM_PROTOTYPES_H
+/*
+ * This file is for prototypes of C functions that are only called
+ * from asm, and any associated variables.
+ *
+ * Copyright 2016, Daniel Axtens, IBM Corporation.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ */
+
+/* Patch sites */
+extern s32 patch__call_flush_count_cache;
+extern s32 patch__flush_count_cache_return;
+
+extern long flush_count_cache;
+
+#endif /* _ASM_POWERPC_ASM_PROTOTYPES_H */
--- a/arch/powerpc/include/asm/security_features.h
+++ b/arch/powerpc/include/asm/security_features.h
@@ -22,6 +22,7 @@ enum stf_barrier_type {

void setup_stf_barrier(void);
void do_stf_barrier_fixups(enum stf_barrier_type types);
+void setup_count_cache_flush(void);

static inline void security_ftr_set(unsigned long feature)
{
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -25,6 +25,7 @@
#include <asm/page.h>
#include <asm/mmu.h>
#include <asm/thread_info.h>
+#include <asm/code-patching-asm.h>
#include <asm/ppc_asm.h>
#include <asm/asm-offsets.h>
#include <asm/cputable.h>
@@ -450,6 +451,57 @@ _GLOBAL(ret_from_kernel_thread)
li r3,0
b .Lsyscall_exit

+#ifdef CONFIG_PPC_BOOK3S_64
+
+#define FLUSH_COUNT_CACHE \
+1: nop; \
+ patch_site 1b, patch__call_flush_count_cache
+
+
+#define BCCTR_FLUSH .long 0x4c400420
+
+.macro nops number
+ .rept \number
+ nop
+ .endr
+.endm
+
+.balign 32
+.global flush_count_cache
+flush_count_cache:
+ /* Save LR into r9 */
+ mflr r9
+
+ .rept 64
+ bl .+4
+ .endr
+ b 1f
+ nops 6
+
+ .balign 32
+ /* Restore LR */
+1: mtlr r9
+ li r9,0x7fff
+ mtctr r9
+
+ BCCTR_FLUSH
+
+2: nop
+ patch_site 2b patch__flush_count_cache_return
+
+ nops 3
+
+ .rept 278
+ .balign 32
+ BCCTR_FLUSH
+ nops 7
+ .endr
+
+ blr
+#else
+#define FLUSH_COUNT_CACHE
+#endif /* CONFIG_PPC_BOOK3S_64 */
+
/*
* This routine switches between two different tasks. The process
* state of one is saved on its kernel stack. Then the state
@@ -513,6 +565,8 @@ BEGIN_FTR_SECTION
END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
#endif

+ FLUSH_COUNT_CACHE
+
#ifdef CONFIG_SMP
/* We need a sync somewhere here to make sure that if the
* previous task gets rescheduled on another CPU, it sees all
--- a/arch/powerpc/kernel/security.c
+++ b/arch/powerpc/kernel/security.c
@@ -10,12 +10,21 @@
#include <linux/seq_buf.h>

#include <asm/debug.h>
+#include <asm/asm-prototypes.h>
+#include <asm/code-patching.h>
#include <asm/security_features.h>
#include <asm/setup.h>


unsigned long powerpc_security_features __read_mostly = SEC_FTR_DEFAULT;

+enum count_cache_flush_type {
+ COUNT_CACHE_FLUSH_NONE = 0x1,
+ COUNT_CACHE_FLUSH_SW = 0x2,
+ COUNT_CACHE_FLUSH_HW = 0x4,
+};
+static enum count_cache_flush_type count_cache_flush_type;
+
bool barrier_nospec_enabled;
static bool no_nospec;

@@ -160,17 +169,29 @@ ssize_t cpu_show_spectre_v2(struct devic
bcs = security_ftr_enabled(SEC_FTR_BCCTRL_SERIALISED);
ccd = security_ftr_enabled(SEC_FTR_COUNT_CACHE_DISABLED);

- if (bcs || ccd) {
+ if (bcs || ccd || count_cache_flush_type != COUNT_CACHE_FLUSH_NONE) {
+ bool comma = false;
seq_buf_printf(&s, "Mitigation: ");

- if (bcs)
+ if (bcs) {
seq_buf_printf(&s, "Indirect branch serialisation (kernel only)");
+ comma = true;
+ }
+
+ if (ccd) {
+ if (comma)
+ seq_buf_printf(&s, ", ");
+ seq_buf_printf(&s, "Indirect branch cache disabled");
+ comma = true;
+ }

- if (bcs && ccd)
+ if (comma)
seq_buf_printf(&s, ", ");

- if (ccd)
- seq_buf_printf(&s, "Indirect branch cache disabled");
+ seq_buf_printf(&s, "Software count cache flush");
+
+ if (count_cache_flush_type == COUNT_CACHE_FLUSH_HW)
+ seq_buf_printf(&s, "(hardware accelerated)");
} else
seq_buf_printf(&s, "Vulnerable");

@@ -325,4 +346,71 @@ static __init int stf_barrier_debugfs_in
}
device_initcall(stf_barrier_debugfs_init);
#endif /* CONFIG_DEBUG_FS */
+
+static void toggle_count_cache_flush(bool enable)
+{
+ if (!enable || !security_ftr_enabled(SEC_FTR_FLUSH_COUNT_CACHE)) {
+ patch_instruction_site(&patch__call_flush_count_cache, PPC_INST_NOP);
+ count_cache_flush_type = COUNT_CACHE_FLUSH_NONE;
+ pr_info("count-cache-flush: software flush disabled.\n");
+ return;
+ }
+
+ patch_branch_site(&patch__call_flush_count_cache,
+ (u64)&flush_count_cache, BRANCH_SET_LINK);
+
+ if (!security_ftr_enabled(SEC_FTR_BCCTR_FLUSH_ASSIST)) {
+ count_cache_flush_type = COUNT_CACHE_FLUSH_SW;
+ pr_info("count-cache-flush: full software flush sequence enabled.\n");
+ return;
+ }
+
+ patch_instruction_site(&patch__flush_count_cache_return, PPC_INST_BLR);
+ count_cache_flush_type = COUNT_CACHE_FLUSH_HW;
+ pr_info("count-cache-flush: hardware assisted flush sequence enabled\n");
+}
+
+void setup_count_cache_flush(void)
+{
+ toggle_count_cache_flush(true);
+}
+
+#ifdef CONFIG_DEBUG_FS
+static int count_cache_flush_set(void *data, u64 val)
+{
+ bool enable;
+
+ if (val == 1)
+ enable = true;
+ else if (val == 0)
+ enable = false;
+ else
+ return -EINVAL;
+
+ toggle_count_cache_flush(enable);
+
+ return 0;
+}
+
+static int count_cache_flush_get(void *data, u64 *val)
+{
+ if (count_cache_flush_type == COUNT_CACHE_FLUSH_NONE)
+ *val = 0;
+ else
+ *val = 1;
+
+ return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_count_cache_flush, count_cache_flush_get,
+ count_cache_flush_set, "%llu\n");
+
+static __init int count_cache_flush_debugfs_init(void)
+{
+ debugfs_create_file("count_cache_flush", 0600, powerpc_debugfs_root,
+ NULL, &fops_count_cache_flush);
+ return 0;
+}
+device_initcall(count_cache_flush_debugfs_init);
+#endif /* CONFIG_DEBUG_FS */
#endif /* CONFIG_PPC_BOOK3S_64 */

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