Explicitly disallow enabling mitigations at runtime for kernels that were
built with CONFIG_SPECULATION_MITIGATIONS=n.  Because more Kconfigs are
buried behind SPECULATION_MITIGATIONS, 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                                | 10 +++++++---
 kernel/cpu.c                                    |  2 ++
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 70046a019d42..7d623df11a1a 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 on x86 if and only if
+			the kernel was built with SPECULATION_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 10a6251f58f3..f4e4dd360636 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2493,10 +2493,14 @@ menuconfig SPECULATION_MITIGATIONS
 	default y
 	help
 	  Say Y here to enable options which enable mitigations for
-	  speculative execution hardware vulnerabilities.
+	  speculative execution hardware vulnerabilities.  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 SPECULATION_MITIGATIONS
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 07ad53b7f119..d445763d8047 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3214,6 +3214,8 @@ static int __init mitigations_parse_cmdline(char *arg)
 {
 	if (!strcmp(arg, "off"))
 		cpu_mitigations = CPU_MITIGATIONS_OFF;
+	else if (!IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS))
+		pr_crit("Kernel compiled without mitigations, system may still be vulnerable\n");
 	else if (!strcmp(arg, "auto"))
 		cpu_mitigations = CPU_MITIGATIONS_AUTO;
 	else if (!strcmp(arg, "auto,nosmt"))
-- 
2.44.0.478.gd926399ef9-goog

* Sean Christopherson <seanjc@google.com> wrote:

> Explicitly disallow enabling mitigations at runtime for kernels that were
> built with CONFIG_SPECULATION_MITIGATIONS=n.  Because more Kconfigs are
> buried behind SPECULATION_MITIGATIONS, 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                                | 10 +++++++---
>  kernel/cpu.c                                    |  2 ++
>  3 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 70046a019d42..7d623df11a1a 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 on x86 if and only if
> +			the kernel was built with SPECULATION_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 10a6251f58f3..f4e4dd360636 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2493,10 +2493,14 @@ menuconfig SPECULATION_MITIGATIONS
>  	default y
>  	help
>  	  Say Y here to enable options which enable mitigations for
> -	  speculative execution hardware vulnerabilities.
> +	  speculative execution hardware vulnerabilities.  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 SPECULATION_MITIGATIONS
>  
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 07ad53b7f119..d445763d8047 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3214,6 +3214,8 @@ static int __init mitigations_parse_cmdline(char *arg)
>  {
>  	if (!strcmp(arg, "off"))
>  		cpu_mitigations = CPU_MITIGATIONS_OFF;
> +	else if (!IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS))
> +		pr_crit("Kernel compiled without mitigations, system may still be vulnerable\n");

This doesn't really make it clear that the kernel is actively ignoring the 
mitigations= command line. I think something like this would be more clear:

> +		pr_crit("Kernel compiled without mitigations, ignoring mitigations= boot option. System may still be vulnerable\n");

Thanks,

	Ingo