[PATCH] x86/speculation: Clean up various Spectre related details

Ingo Molnar
2018-02-11T18:51:04Z

* David Woodhouse <dwmw@amazon.co.uk> wrote:

> + /*
> + * Retpoline means the kernel is safe because it has no indirect
> + * branches. But firmware isn't, so use IBRS to protect that.
> + */
> + if (boot_cpu_has(X86_FEATURE_IBRS)) {
> + setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
> + pr_info("Enabling Restricted Speculation for firmware calls\n");
> + }

I have changed this text to say:

pr_info("Spectre mitigation: Restricting branch speculation (enabling IBRS) for firmware calls\n");

In fact while at it I found and improved a few other details as well, such as:

* Retpoline means the kernel is safe because it has no indirect
- * branches. But firmware isn't, so use IBRS to protect that.
+ * branches. But we don't know whether the firmware is safe, so
+ * use IBRS to protect against that:

most Spectre related messages are now harmonized:

arch/x86/kernel/cpu/bugs.c: pr_info("Spectre mitigation: Filling RSB on context switch\n");
arch/x86/kernel/cpu/bugs.c: pr_info("Spectre mitigation: Enabling Indirect Branch Prediction Barrier (IBPB)\n");
arch/x86/kernel/cpu/bugs.c: pr_info("Spectre mitigation: Restricting branch speculation (enabling IBRS) for firmware calls\n");

Find the full patch below.

Thanks,

Ingo

=========================>
>From 82c2b2f29691143a05181333f387e786646aa28b Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Sat, 10 Feb 2018 11:51:57 +0100
Subject: [PATCH] x86/speculation: Clean up various Spectre related details

Harmonize all the Spectre messages so that a:

dmesg | grep -i spectre

... gives us most Spectre related kernel boot messages.

Also fix a few other details:

- clarify a comment about firmware speculation control

- s/KPTI/PTI

- remove various line-breaks that made the code uglier

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/kernel/cpu/bugs.c | 33 ++++++++++++++-------------------
1 file changed, 14 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6f6d763225c8..eff45477fcca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -162,8 +162,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
if (cmdline_find_option_bool(boot_command_line, "nospectre_v2"))
return SPECTRE_V2_CMD_NONE;
else {
- ret = cmdline_find_option(boot_command_line, "spectre_v2", arg,
- sizeof(arg));
+ ret = cmdline_find_option(boot_command_line, "spectre_v2", arg, sizeof(arg));
if (ret < 0)
return SPECTRE_V2_CMD_AUTO;

@@ -175,8 +174,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
}

if (i >= ARRAY_SIZE(mitigation_options)) {
- pr_err("unknown option (%s). Switching to AUTO select\n",
- mitigation_options[i].option);
+ pr_err("unknown option (%s). Switching to AUTO select\n", mitigation_options[i].option);
return SPECTRE_V2_CMD_AUTO;
}
}
@@ -185,8 +183,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
cmd == SPECTRE_V2_CMD_RETPOLINE_AMD ||
cmd == SPECTRE_V2_CMD_RETPOLINE_GENERIC) &&
!IS_ENABLED(CONFIG_RETPOLINE)) {
- pr_err("%s selected but not compiled in. Switching to AUTO select\n",
- mitigation_options[i].option);
+ pr_err("%s selected but not compiled in. Switching to AUTO select\n", mitigation_options[i].option);
return SPECTRE_V2_CMD_AUTO;
}

@@ -256,14 +253,14 @@ static void __init spectre_v2_select_mitigation(void)
goto retpoline_auto;
break;
}
- pr_err("kernel not compiled with retpoline; no mitigation available!");
+ pr_err("Spectre mitigation: kernel not compiled with retpoline; no mitigation available!");
return;

retpoline_auto:
if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
retpoline_amd:
if (!boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
- pr_err("LFENCE not serializing. Switching to generic retpoline\n");
+ pr_err("Spectre mitigation: LFENCE not serializing, switching to generic retpoline\n");
goto retpoline_generic;
}
mode = retp_compiler() ? SPECTRE_V2_RETPOLINE_AMD :
@@ -281,7 +278,7 @@ static void __init spectre_v2_select_mitigation(void)
pr_info("%s\n", spectre_v2_strings[mode]);

/*
- * If neither SMEP or KPTI are available, there is a risk of
+ * If neither SMEP or PTI are available, there is a risk of
* hitting userspace addresses in the RSB after a context switch
* from a shallow call stack to a deeper one. To prevent this fill
* the entire RSB, even when using IBRS.
@@ -295,30 +292,30 @@ static void __init spectre_v2_select_mitigation(void)
if ((!boot_cpu_has(X86_FEATURE_PTI) &&
!boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) {
setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
- pr_info("Filling RSB on context switch\n");
+ pr_info("Spectre mitigation: Filling RSB on context switch\n");
}

/* Initialize Indirect Branch Prediction Barrier if supported */
if (boot_cpu_has(X86_FEATURE_IBPB)) {
setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
- pr_info("Enabling Indirect Branch Prediction Barrier\n");
+ pr_info("Spectre mitigation: Enabling Indirect Branch Prediction Barrier (IBPB)\n");
}

/*
* Retpoline means the kernel is safe because it has no indirect
- * branches. But firmware isn't, so use IBRS to protect that.
+ * branches. But we don't know whether the firmware is safe, so
+ * use IBRS to protect against that:
*/
if (boot_cpu_has(X86_FEATURE_IBRS)) {
setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
- pr_info("Enabling Restricted Speculation for firmware calls\n");
+ pr_info("Spectre mitigation: Restricting branch speculation (enabling IBRS) for firmware calls\n");
}
}

#undef pr_fmt

#ifdef CONFIG_SYSFS
-ssize_t cpu_show_meltdown(struct device *dev,
- struct device_attribute *attr, char *buf)
+ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr, char *buf)
{
if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
return sprintf(buf, "Not affected\n");
@@ -327,16 +324,14 @@ ssize_t cpu_show_meltdown(struct device *dev,
return sprintf(buf, "Vulnerable\n");
}

-ssize_t cpu_show_spectre_v1(struct device *dev,
- struct device_attribute *attr, char *buf)
+ssize_t cpu_show_spectre_v1(struct device *dev, struct device_attribute *attr, char *buf)
{
if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V1))
return sprintf(buf, "Not affected\n");
return sprintf(buf, "Mitigation: __user pointer sanitization\n");
}

-ssize_t cpu_show_spectre_v2(struct device *dev,
- struct device_attribute *attr, char *buf)
+ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr, char *buf)
{
if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
return sprintf(buf, "Not affected\n");