[PATCH 5.18 52/61] KVM: VMX: Prevent RSB underflow before vmenter

Greg Kroah-Hartman
2022-07-12T19:15:10Z

From: Josh Poimboeuf <jpoimboe@kernel.org>

commit 07853adc29a058c5fd143c14e5ac528448a72ed9 upstream.

On VMX, there are some balanced returns between the time the guest's
SPEC_CTRL value is written, and the vmenter.

Balanced returns (matched by a preceding call) are usually ok, but it's
at least theoretically possible an NMI with a deep call stack could
empty the RSB before one of the returns.

For maximum paranoia, don't allow *any* returns (balanced or otherwise)
between the SPEC_CTRL write and the vmenter.

[ bp: Fix 32-bit build. ]

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
[cascardo: header conflict fixup at arch/x86/kernel/asm-offsets.c]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kernel/asm-offsets.c | 6 ++++++
arch/x86/kernel/cpu/bugs.c | 4 ++--
arch/x86/kvm/vmx/capabilities.h | 4 ++--
arch/x86/kvm/vmx/vmenter.S | 29 +++++++++++++++++++++++++++++
arch/x86/kvm/vmx/vmx.c | 8 --------
arch/x86/kvm/vmx/vmx.h | 4 ++--
arch/x86/kvm/vmx/vmx_ops.h | 2 +-
7 files changed, 42 insertions(+), 15 deletions(-)

--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -18,6 +18,7 @@
#include <asm/bootparam.h>
#include <asm/suspend.h>
#include <asm/tlbflush.h>
+#include "../kvm/vmx/vmx.h"

#ifdef CONFIG_XEN
#include <xen/interface/xen.h>
@@ -90,4 +91,9 @@ static void __used common(void)
OFFSET(TSS_sp0, tss_struct, x86_tss.sp0);
OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
+
+ if (IS_ENABLED(CONFIG_KVM_INTEL)) {
+ BLANK();
+ OFFSET(VMX_spec_ctrl, vcpu_vmx, spec_ctrl);
+ }
}
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -196,8 +196,8 @@ void __init check_bugs(void)
}

/*
- * NOTE: For VMX, this function is not called in the vmexit path.
- * It uses vmx_spec_ctrl_restore_host() instead.
+ * NOTE: This function is *only* called for SVM. VMX spec_ctrl handling is
+ * done in vmenter.S.
*/
void
x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -4,8 +4,8 @@

#include <asm/vmx.h>

-#include "lapic.h"
-#include "x86.h"
+#include "../lapic.h"
+#include "../x86.h"

extern bool __read_mostly enable_vpid;
extern bool __read_mostly flexpriority_enabled;
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -1,9 +1,11 @@
/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/linkage.h>
#include <asm/asm.h>
+#include <asm/asm-offsets.h>
#include <asm/bitsperlong.h>
#include <asm/kvm_vcpu_regs.h>
#include <asm/nospec-branch.h>
+#include <asm/percpu.h>
#include <asm/segment.h>
#include "run_flags.h"

@@ -73,6 +75,33 @@ SYM_FUNC_START(__vmx_vcpu_run)
lea (%_ASM_SP), %_ASM_ARG2
call vmx_update_host_rsp

+ ALTERNATIVE "jmp .Lspec_ctrl_done", "", X86_FEATURE_MSR_SPEC_CTRL
+
+ /*
+ * SPEC_CTRL handling: if the guest's SPEC_CTRL value differs from the
+ * host's, write the MSR.
+ *
+ * IMPORTANT: To avoid RSB underflow attacks and any other nastiness,
+ * there must not be any returns or indirect branches between this code
+ * and vmentry.
+ */
+ mov 2*WORD_SIZE(%_ASM_SP), %_ASM_DI
+ movl VMX_spec_ctrl(%_ASM_DI), %edi
+ movl PER_CPU_VAR(x86_spec_ctrl_current), %esi
+ cmp %edi, %esi
+ je .Lspec_ctrl_done
+ mov $MSR_IA32_SPEC_CTRL, %ecx
+ xor %edx, %edx
+ mov %edi, %eax
+ wrmsr
+
+.Lspec_ctrl_done:
+
+ /*
+ * Since vmentry is serializing on affected CPUs, there's no need for
+ * an LFENCE to stop speculation from skipping the wrmsr.
+ */
+
/* Load @regs to RAX. */
mov (%_ASM_SP), %_ASM_AX

--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6989,14 +6989,6 @@ static fastpath_t vmx_vcpu_run(struct kv

kvm_wait_lapic_expire(vcpu);

- /*
- * If this vCPU has touched SPEC_CTRL, restore the guest's value if
- * it's non-zero. Since vmentry is serialising on affected CPUs, there
- * is no need to worry about the conditional branch over the wrmsr
- * being speculatively taken.
- */
- x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
-
/* The actual VMENTER/EXIT is in the .noinstr.text section. */
vmx_vcpu_enter_exit(vcpu, vmx, __vmx_vcpu_run_flags(vmx));

--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -8,11 +8,11 @@
#include <asm/intel_pt.h>

#include "capabilities.h"
-#include "kvm_cache_regs.h"
+#include "../kvm_cache_regs.h"
#include "posted_intr.h"
#include "vmcs.h"
#include "vmx_ops.h"
-#include "cpuid.h"
+#include "../cpuid.h"
#include "run_flags.h"

#define MSR_TYPE_R 1
--- a/arch/x86/kvm/vmx/vmx_ops.h
+++ b/arch/x86/kvm/vmx/vmx_ops.h
@@ -8,7 +8,7 @@

#include "evmcs.h"
#include "vmcs.h"
-#include "x86.h"
+#include "../x86.h"

asmlinkage void vmread_error(unsigned long field, bool fault);
__attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,