x86/bhi: Mitigate KVM by default

commit 95a6ccbdc7199a14b71ad8901cb788ba7fb5167b upstream.

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: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index 62c7902c66fd6eb11378a486394a99d8ae4ef9f7..9edb2860a3e1981acc256f6b242013becdde7f44 100644 (file)
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -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
 

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 729d6e5916dc6581bacf9e2b48178d345098cf9a..95efaccca4539530fc534b94b7c4fe1de155d5e0 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6039,8 +6039,9 @@
 
                        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] Control mitigation of Spectre variant 2
                        (indirect branch speculation) vulnerability.

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index dc510a4e6aad6e85c88796c5097480c39825122a..76b3b00ff1021e6cd54730ccba9a4a0cb6f83000 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -465,6 +465,7 @@
 #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)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index fb8a530d6c1813e1bf7a3cf454e5ff20ffde8dff..3ff081a74ab2b47546560bed07ba91e207ebd54f 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -339,8 +339,13 @@
 .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__ */

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 22c5de50be2efa4f13b3a9d27442217164f350c6..0d7238d88b38a0aa592533e969abb353e6f737f5 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1667,9 +1667,14 @@ 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");
 }
@@ -2810,10 +2815,12 @@ 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)";
 }

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 0f3593e10c57d850db0c4875ec817915beebd4b9..f6986dee6f8c7c52622857f131adf766d1528121 100644 (file)
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -275,7 +275,7 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_GLOBAL)
 
        call vmx_spec_ctrl_restore_host
 
-       CLEAR_BRANCH_HISTORY
+       CLEAR_BRANCH_HISTORY_VMEXIT
 
        /* Put return value in AX */
        mov %_ASM_BX, %_ASM_AX