x86/bugs: Add 'spectre_bhi=vmexit' cmdline option

In cloud environments it can be useful to *only* enable the vmexit
mitigation and leave syscalls vulnerable.  Add that as an option.

This is similar to the old spectre_bhi=auto option which was removed
with the following commit:

  36d4fe1 ("x86/bugs: Remove CONFIG_BHI_MITIGATION_AUTO and spectre_bhi=auto")

with the main difference being that this has a more descriptive name and
is disabled by default.

Mitigation switch requested by Maksim Davydov <davydov-max@yandex-team.ru>.

  [ bp: Massage. ]

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/2cbad706a6d5e1da2829e5e123d8d5c80330148c.1719381528.git.jpoimboe@kernel.org

Author: jpoimboe

Documentation/admin-guide/kernel-parameters.txt

@@ -6136,9 +6136,15 @@
 			deployment of the HW BHI control and the SW BHB
 			clearing sequence.
 
-			on   - (default) Enable the HW or SW mitigation
-			       as needed.
-			off  - Disable the mitigation.
+			on     - (default) Enable the HW or SW mitigation as
+				 needed.  This protects the kernel from
+				 both syscalls and VMs.
+			vmexit - 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.
+			off    - Disable the mitigation.
 
 	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.

arch/x86/kernel/cpu/bugs.c

@@ -1625,6 +1625,7 @@ static bool __init spec_ctrl_bhi_dis(void)
 enum bhi_mitigations {
 	BHI_MITIGATION_OFF,
 	BHI_MITIGATION_ON,
+	BHI_MITIGATION_VMEXIT_ONLY,
 };
 
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
@@ -1639,6 +1640,8 @@ static int __init spectre_bhi_parse_cmdline(char *str)
 		bhi_mitigation = BHI_MITIGATION_OFF;
 	else if (!strcmp(str, "on"))
 		bhi_mitigation = BHI_MITIGATION_ON;
+	else if (!strcmp(str, "vmexit"))
+		bhi_mitigation = BHI_MITIGATION_VMEXIT_ONLY;
 	else
 		pr_err("Ignoring unknown spectre_bhi option (%s)", str);
 
@@ -1659,19 +1662,22 @@ static void __init bhi_select_mitigation(void)
 			return;
 	}
 
+	/* Mitigate in hardware if supported */
 	if (spec_ctrl_bhi_dis())
 		return;
 
 	if (!IS_ENABLED(CONFIG_X86_64))
 		return;
 
-	/* Mitigate KVM by default */
-	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
-	pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
+	if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
+		pr_info("Spectre BHI mitigation: SW BHB clearing on VM exit only\n");
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
+		return;
+	}
 
-	/* Mitigate syscalls when the mitigation is forced =on */
+	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall and VM exit\n");
 	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
-	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
+	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
 }
 
 static void __init spectre_v2_select_mitigation(void)