Commit 42c141fb authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Borislav Petkov (AMD)
Browse files

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:

  36d4fe14 ("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: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: default avatarDaniel Sneddon <daniel.sneddon@linux.intel.com>
Reviewed-by: default avatarNikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/2cbad706a6d5e1da2829e5e123d8d5c80330148c.1719381528.git.jpoimboe@kernel.org
parent 4586c93e

Documentation/admin-guide/kernel-parameters.txt

+9 −3
Original line number Diff line number Diff line
@@ -6136,8 +6136,14 @@ 
			deployment of the HW BHI control and the SW BHB

			clearing sequence.



			on   - (default) Enable the HW or SW mitigation

			       as needed.

			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

arch/x86/kernel/cpu/bugs.c

+11 −5
Original line number Diff line number Diff line
@@ -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 */

	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);

	pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");

		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)