Re: [PATCH v5 3/3] x86/bugs: Add 'spectre_bhi=vmexit' cmdline option

[Josh Poimboeuf]

On Mon, May 27, 2024 at 01:45:59PM +0300, Maksim Davydov wrote:
> I think it will be useful for us to have appropriate Kconfig option. Could
> you please add it to the next version?

That should probably be a separate patch, something like the below?

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1d7122a1883e..ab1ea701bc42 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2642,17 +2642,46 @@ config MITIGATION_RFDS
 	  stored in floating point, vector and integer registers.
 	  See also <file:Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst>
 
-config MITIGATION_SPECTRE_BHI
-	bool "Mitigate Spectre-BHB (Branch History Injection)"
+choice
+	prompt "Mitigate Spectre-BHB (Branch History Injection)"
 	depends on CPU_SUP_INTEL
-	default y
+	default MITIGATION_SPECTRE_BHI_ON
 	help
 	  Enable BHI mitigations. BHI attacks are a form of Spectre V2 attacks
 	  where the branch history buffer is poisoned to speculatively steer
 	  indirect branches.
+
+	  The compile-time default can be set to on, vmexit, or off,
+	  corresponding to the "spectre_bhi=" cmdline defaults described in
+	  Documentation/admin-guide/kernel-parameters.rst.  The cmdline
+	  options can be used to override this compile-time default.
+
 	  See <file:Documentation/admin-guide/hw-vuln/spectre.rst>
 
-endif
+config MITIGATION_SPECTRE_BHI_ON
+	bool "on"
+	help
+	  Enable the HW or SW mitigation as needed.  This protects the kernel
+	  from both syscalls and VMs.  Equivalent to the spectre_bhi=on cmdline
+	  option.
+
+config MITIGATION_SPECTRE_BHI_VMEXIT
+	bool "vmexit"
+	help
+	  On systems which don't have the HW mitigation available, enable the
+	  SW mitigation on vmexit ONLY.  On such systems, the host kernel is
+	  protected from VM-originated BHI attacks, but may still be vulnerable
+	  to syscall attacks.  Equivalent to the spectre_bhi=vmexit cmdline
+	  option.
+
+config MITIGATION_SPECTRE_BHI_OFF
+	bool "off"
+	help
+	  Disable the mitigation.  Equivalent to the spectre_bhi=off cmdline
+	  option.
+endchoice
+
+endif # CPU_MITIGATIONS
 
 config ARCH_HAS_ADD_PAGES
 	def_bool y
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 94bcf29df465..d415f24b7169 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1628,8 +1628,13 @@ enum bhi_mitigations {
 	BHI_MITIGATION_VMEXIT_ONLY,
 };
 
-static enum bhi_mitigations bhi_mitigation __ro_after_init =
-	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
+#ifdef CONFIG_MITIGATION_SPECTRE_BHI_ON
+static enum bhi_mitigations bhi_mitigation __ro_after_init = BHI_MITIGATION_ON;
+#elif CONFIG_MITIGATION_SPECTRE_BHI_VMEXIT
+static enum bhi_mitigations bhi_mitigation __ro_after_init = BHI_MITIGATION_VMEXIT;
+#else
+static enum bhi_mitigations bhi_mitigation __ro_after_init = BHI_MITIGATION_OFF;
+#endif
 
 static int __init spectre_bhi_parse_cmdline(char *str)
 {