From: Sean Christopherson <seanjc@google.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
Stephen Rothwell <sfr@canb.auug.org.au>,
Michael Ellerman <mpe@ellerman.id.au>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Josh Poimboeuf <jpoimboe@kernel.org>,
Will Deacon <will@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Sean Christopherson <seanjc@google.com>
Subject: [PATCH v2 2/2] cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n
Date: Fri, 19 Apr 2024 17:05:55 -0700 [thread overview]
Message-ID: <20240420000556.2645001-3-seanjc@google.com> (raw)
In-Reply-To: <20240420000556.2645001-1-seanjc@google.com>
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 requrie build-time support, BHI mitigations will
still be off without additional kernel parameters, etc.
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +++
arch/x86/Kconfig | 8 ++++++--
include/linux/cpu.h | 11 +++++++++++
kernel/cpu.c | 13 ++++++++++---
4 files changed, 30 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 902ecd92a29f..213d0719e2b7 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 619a04d5c131..928820e61cb5 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 272e4e79e15c..ee0a3b4e0769 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 false;
+}
+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 bb0ff275fb46..24235c1d6e82 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.
@@ -3207,8 +3208,7 @@ enum cpu_mitigations {
};
static enum cpu_mitigations cpu_mitigations __ro_after_init =
- IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
- CPU_MITIGATIONS_OFF;
+ CPU_MITIGATIONS_AUTO;
static int __init mitigations_parse_cmdline(char *arg)
{
@@ -3224,7 +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)
@@ -3239,3 +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);
--
2.44.0.769.g3c40516874-goog
next prev parent reply other threads:[~2024-04-20 0:06 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 ` Sean Christopherson [this message]
2024-04-20 13:36 ` [PATCH v2 2/2] cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n 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: x86/urgent] " tip-bot2 for Sean Christopherson
2024-04-20 13:33 ` [PATCH v2 0/2] cpu: Fix default mitigation behavior Borislav Petkov
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20240420000556.2645001-3-seanjc@google.com \
--to=seanjc@google.com \
--cc=bp@alien8.de \
--cc=dave.hansen@linux.intel.com \
--cc=geert@linux-m68k.org \
--cc=jpoimboe@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=mpe@ellerman.id.au \
--cc=peterz@infradead.org \
--cc=sfr@canb.auug.org.au \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
--cc=will@kernel.org \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.