Commit 95a6ccbd authored by Pawan Gupta's avatar Pawan Gupta Committed by Thomas Gleixner
Browse files

x86/bhi: Mitigate KVM by default 



BHI mitigation mode spectre_bhi=auto does not deploy the software
mitigation by default. In a cloud environment, it is a likely scenario
where userspace is trusted but the guests are not trusted. Deploying
system wide mitigation in such cases is not desirable.

Update the auto mode to unconditionally mitigate against malicious
guests. Deploy the software sequence at VMexit in auto mode also, when
hardware mitigation is not available. Unlike the force =on mode,
software sequence is not deployed at syscalls in auto mode.

Suggested-by: default avatarAlexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: default avatarPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: default avatarDaniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarAlexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
parent ec9404e4

Documentation/admin-guide/hw-vuln/spectre.rst

+5 −2
Original line number Diff line number Diff line
@@ -439,10 +439,12 @@ The possible values in this file are: 
   - System is protected by retpoline

 * - BHI: BHI_DIS_S

   - System is protected by BHI_DIS_S

 * - BHI: SW loop

 * - BHI: SW loop; KVM SW loop

   - System is protected by software clearing sequence

 * - BHI: Syscall hardening

   - Syscalls are hardened against BHI

 * - BHI: Syscall hardening; KVM: SW loop

   - System is protected from userspace attacks by syscall hardening; KVM is protected by software clearing sequence



Full mitigation might require a microcode update from the CPU

vendor. When the necessary microcode is not available, the kernel will
@@ -669,7 +671,8 @@ kernel command line. 
			unconditionally disable.

		auto

			enable if hardware mitigation

			control(BHI_DIS_S) is available.

			control(BHI_DIS_S) is available, otherwise

			enable alternate mitigation in KVM.



For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt

Documentation/admin-guide/kernel-parameters.txt

+3 −2
Original line number Diff line number Diff line
@@ -6071,8 +6071,9 @@ 


			on   - unconditionally enable.

			off  - unconditionally disable.

			auto - (default) enable only if hardware mitigation

			       control(BHI_DIS_S) is available.

			auto - (default) enable hardware mitigation

			       (BHI_DIS_S) if available, otherwise enable

			       alternate mitigation in KVM.



	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2

			(indirect branch speculation) vulnerability.

arch/x86/include/asm/cpufeatures.h

+1 −0
Original line number Diff line number Diff line
@@ -469,6 +469,7 @@ 
#define X86_FEATURE_CLEAR_BHB_LOOP	(21*32+ 1) /* "" Clear branch history at syscall entry using SW loop */

#define X86_FEATURE_BHI_CTRL		(21*32+ 2) /* "" BHI_DIS_S HW control available */

#define X86_FEATURE_CLEAR_BHB_HW	(21*32+ 3) /* "" BHI_DIS_S HW control enabled */

#define X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT (21*32+ 4) /* "" Clear branch history at vmexit using SW loop */



/*

 * BUG word(s)

arch/x86/include/asm/nospec-branch.h

+5 −0
Original line number Diff line number Diff line
@@ -330,8 +330,13 @@ 
.macro CLEAR_BRANCH_HISTORY

	ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP

.endm



.macro CLEAR_BRANCH_HISTORY_VMEXIT

	ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT

.endm

#else

#define CLEAR_BRANCH_HISTORY

#define CLEAR_BRANCH_HISTORY_VMEXIT

#endif



#else /* __ASSEMBLY__ */

arch/x86/kernel/cpu/bugs.c

+8 −1
Original line number Diff line number Diff line
@@ -1668,9 +1668,14 @@ static void __init bhi_select_mitigation(void) 
	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_AUTO)

		return;



	/* Mitigate syscalls when the mitigation is forced =on */

	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);

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

}
@@ -2811,10 +2816,12 @@ static const char * const spectre_bhi_state(void) 
	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_HW))

		return "; BHI: BHI_DIS_S";

	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))

		return "; BHI: SW loop";

		return "; BHI: SW loop, KVM: SW loop";

	else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&

		 !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))

		return "; BHI: Retpoline";

	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))

		return "; BHI: Syscall hardening, KVM: SW loop";



	return "; BHI: Vulnerable (Syscall hardening enabled)";

}