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 6e70c0d433317465c259f09a2a43d901869d3e41..61199466c0437357efd81582a272c06a39c7b71b 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5928,8 +5928,9 @@
 
                        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 e4673b73e9e01986094fc9f8831c2924c158433e..8c1593dd2c3171f019a37a18166f20fa2faf0439 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -466,6 +466,7 @@
 #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 76a0419956e68a672a52ad97dddd152931dba841..a8781c8763b44ffa57fc9d7413ea8250045dc625 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 855500318d75d3a4c76227a12d3c55271e0e8a61..3fc23015562711f64927ff59790224e039ad590e 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 040bd8e5670baca6bd6746e48278ffc7ba7867e4..9522d46567f81bb63940301cd40ce968ae594de0 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