From: Sasha Levin <sashal@kernel.org>
To: linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc: Sean Christopherson <seanjc@google.com>,
Borislav Petkov <bp@alien8.de>, Sasha Levin <sashal@kernel.org>,
corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com,
dave.hansen@linux.intel.com, x86@kernel.org,
peterz@infradead.org, paulmck@kernel.org, jpoimboe@kernel.org,
tj@kernel.org, xiongwei.song@windriver.com,
linux-doc@vger.kernel.org
Subject: [PATCH AUTOSEL 6.6 15/19] cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n
Date: Tue, 7 May 2024 18:58:37 -0400 [thread overview]
Message-ID: <20240507225910.390914-15-sashal@kernel.org> (raw)
In-Reply-To: <20240507225910.390914-1-sashal@kernel.org>
From: Sean Christopherson <seanjc@google.com>
[ Upstream commit ce0abef6a1d540acef85068e0e82bdf1fbeeb0e9 ]
Explicitly disallow enabling mitigations at runtime for kernels that were
built with CONFIG_CPU_MITIGATIONS=n, as some architectures may omit code
entirely if mitigations are disabled at compile time.
E.g. on x86, a large pile of Kconfigs are buried behind CPU_MITIGATIONS,
and trying to provide sane behavior for retroactively enabling mitigations
is extremely difficult, bordering on impossible. E.g. page table isolation
and call depth tracking require build-time support, BHI mitigations will
still be off without additional kernel parameters, etc.
[ bp: Touchups. ]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240420000556.2645001-3-seanjc@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +++
arch/x86/Kconfig | 8 ++++++--
include/linux/cpu.h | 11 +++++++++++
kernel/cpu.c | 14 ++++++++++----
4 files changed, 30 insertions(+), 6 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 66dfc348043d6..8d2f9ed3f1076 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3323,6 +3323,9 @@
arch-independent options, each of which is an
aggregation of existing arch-specific options.
+ Note, "mitigations" is supported if and only if the
+ kernel was built with CPU_MITIGATIONS=y.
+
off
Disable all optional CPU mitigations. This
improves system performance, but it may also
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be9248e5cb71b..82d12c93feabe 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2428,9 +2428,13 @@ menuconfig CPU_MITIGATIONS
help
Say Y here to enable options which enable mitigations for hardware
vulnerabilities (usually related to speculative execution).
+ Mitigations can be disabled or restricted to SMT systems at runtime
+ via the "mitigations" kernel parameter.
- If you say N, all mitigations will be disabled. You really
- should know what you are doing to say so.
+ If you say N, all mitigations will be disabled. This CANNOT be
+ overridden at runtime.
+
+ Say 'Y', unless you really know what you are doing.
if CPU_MITIGATIONS
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index e990c180282e7..a7d91a167a8b6 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -214,7 +214,18 @@ void cpuhp_report_idle_dead(void);
static inline void cpuhp_report_idle_dead(void) { }
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
+#ifdef CONFIG_CPU_MITIGATIONS
extern bool cpu_mitigations_off(void);
extern bool cpu_mitigations_auto_nosmt(void);
+#else
+static inline bool cpu_mitigations_off(void)
+{
+ return true;
+}
+static inline bool cpu_mitigations_auto_nosmt(void)
+{
+ return false;
+}
+#endif
#endif /* _LINUX_CPU_H_ */
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2dd2fd300e916..4f453226fcf48 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3197,6 +3197,7 @@ void __init boot_cpu_hotplug_init(void)
this_cpu_write(cpuhp_state.target, CPUHP_ONLINE);
}
+#ifdef CONFIG_CPU_MITIGATIONS
/*
* These are used for a global "mitigations=" cmdline option for toggling
* optional CPU mitigations.
@@ -3207,9 +3208,7 @@ enum cpu_mitigations {
CPU_MITIGATIONS_AUTO_NOSMT,
};
-static enum cpu_mitigations cpu_mitigations __ro_after_init =
- IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
- CPU_MITIGATIONS_OFF;
+static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
static int __init mitigations_parse_cmdline(char *arg)
{
@@ -3225,7 +3224,6 @@ static int __init mitigations_parse_cmdline(char *arg)
return 0;
}
-early_param("mitigations", mitigations_parse_cmdline);
/* mitigations=off */
bool cpu_mitigations_off(void)
@@ -3240,3 +3238,11 @@ bool cpu_mitigations_auto_nosmt(void)
return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
}
EXPORT_SYMBOL_GPL(cpu_mitigations_auto_nosmt);
+#else
+static int __init mitigations_parse_cmdline(char *arg)
+{
+ pr_crit("Kernel compiled without mitigations, ignoring 'mitigations'; system may still be vulnerable\n");
+ return 0;
+}
+#endif
+early_param("mitigations", mitigations_parse_cmdline);
--
2.43.0
parent reply other threads:[~2024-05-07 23:00 UTC|newest]
Thread overview: expand[flat|nested] mbox.gz Atom feed
[parent not found: <20240507225910.390914-1-sashal@kernel.org>]
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