From: "tip-bot2 for Sean Christopherson" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: Sean Christopherson <seanjc@google.com>,
"Borislav Petkov (AMD)" <bp@alien8.de>,
x86@kernel.org, linux-kernel@vger.kernel.org
Subject: [tip: x86/urgent] cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n
Date: Thu, 25 Apr 2024 14:18:05 -0000 [thread overview]
Message-ID: <171405468558.10875.5701614476992826261.tip-bot2@tip-bot2> (raw)
In-Reply-To: <20240420000556.2645001-3-seanjc@google.com>
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: ce0abef6a1d540acef85068e0e82bdf1fbeeb0e9
Gitweb: https://git.kernel.org/tip/ce0abef6a1d540acef85068e0e82bdf1fbeeb0e9
Author: Sean Christopherson <seanjc@google.com>
AuthorDate: Fri, 19 Apr 2024 17:05:55 -07:00
Committer: Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Thu, 25 Apr 2024 15:47:39 +02:00
cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n
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
---
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 902ecd9..213d071 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3423,6 +3423,9 @@
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 619a04d..928820e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2495,9 +2495,13 @@ 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 272e4e7..861c3bf 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -221,7 +221,18 @@ 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 bb0ff27..63447eb 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3196,6 +3196,7 @@ 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.
@@ -3206,9 +3207,7 @@ 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)
{
@@ -3224,7 +3223,6 @@ static int __init mitigations_parse_cmdline(char *arg)
return 0;
}
-early_param("mitigations", mitigations_parse_cmdline);
/* mitigations=off */
bool cpu_mitigations_off(void)
@@ -3239,3 +3237,11 @@ 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);
next prev parent reply other threads:[~2024-04-25 14:18 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-20 0:05 [PATCH v2 0/2] cpu: Fix default mitigation behavior Sean Christopherson
2024-04-20 0:05 ` [PATCH v2 1/2] cpu: Re-enable CPU mitigations by default for !X86 architectures Sean Christopherson
2024-04-24 5:38 ` Josh Poimboeuf
2024-04-25 14:18 ` [tip: x86/urgent] " tip-bot2 for Sean Christopherson
2024-04-20 0:05 ` [PATCH v2 2/2] cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n Sean Christopherson
2024-04-20 13:36 ` Borislav Petkov
2024-04-24 5:39 ` Josh Poimboeuf
2024-04-24 13:42 ` Borislav Petkov
2024-04-24 15:48 ` Sean Christopherson
2024-04-25 14:18 ` tip-bot2 for Sean Christopherson [this message]
2024-04-20 13:33 ` [PATCH v2 0/2] cpu: Fix default mitigation behavior Borislav Petkov
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