From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
commit fc02735b14fff8c6678b521d324ade27b1a3d4cf upstream.
On eIBRS systems, the returns in the vmexit return path from
__vmx_vcpu_run() to vmx_vcpu_run() are exposed to RSB poisoning attacks.
Fix that by moving the post-vmexit spec_ctrl handling to immediately
after the vmexit.
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[ bp: Adjust for the fact that vmexit is in inline assembly ]
Signed-off-by: Suraj Jitindar Singh <surajjs@xxxxxxxxxx>
---
arch/x86/include/asm/nospec-branch.h | 3 +-
arch/x86/kernel/cpu/bugs.c | 4 +++
arch/x86/kvm/vmx.c | 45 ++++++++++++++++++++++++----
3 files changed, 45 insertions(+), 7 deletions(-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index c7cbad1ec034..2d6d5bac4997 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -257,7 +257,7 @@ extern char __indirect_thunk_end[];
* retpoline and IBRS mitigations for Spectre v2 need this; only on future
* CPUs with IBRS_ALL *might* it be avoided.
*/
-static inline void vmexit_fill_RSB(void)
+static __always_inline void vmexit_fill_RSB(void)
{
#ifdef CONFIG_RETPOLINE
unsigned long loops;
@@ -292,6 +292,7 @@ static inline void indirect_branch_prediction_barrier(void)
/* The Intel SPEC CTRL MSR base value cache */
extern u64 x86_spec_ctrl_base;
+extern u64 x86_spec_ctrl_current;
extern void write_spec_ctrl_current(u64 val, bool force);
extern u64 spec_ctrl_current(void);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 5f805013b7f4..1fde42e5be6e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -185,6 +185,10 @@ void __init check_bugs(void)
#endif
}
+/*
+ * NOTE: For VMX, this function is not called in the vmexit path.
+ * It uses vmx_spec_ctrl_restore_host() instead.
+ */
void
x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
{
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 48b40e160e27..539720a8e094 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9770,10 +9770,31 @@ static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, delta_tsc);
}
+u64 __always_inline vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx)
+{
+ u64 guestval, hostval = this_cpu_read(x86_spec_ctrl_current);
+
+ if (!cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL))
+ return 0;
+
+ guestval = __rdmsr(MSR_IA32_SPEC_CTRL);
+
+ /*
+ * If the guest/host SPEC_CTRL values differ, restore the host value.
+ */
+ if (guestval != hostval)
+ native_wrmsrl(MSR_IA32_SPEC_CTRL, hostval);
+
+ barrier_nospec();
+
+ return guestval;
+}
+
static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
unsigned long debugctlmsr, cr3, cr4;
+ u64 spec_ctrl;
/* Record the guest's net vcpu time for enforced NMI injections. */
if (unlikely(!cpu_has_virtual_nmis() &&
@@ -9967,6 +9988,23 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
, "eax", "ebx", "edi", "esi"
#endif
);
+ /*
+ * IMPORTANT: RSB filling and SPEC_CTRL handling must be done before
+ * the first unbalanced RET after vmexit!
+ *
+ * For retpoline, RSB filling is needed to prevent poisoned RSB entries
+ * and (in some cases) RSB underflow.
+ *
+ * eIBRS has its own protection against poisoned RSB, so it doesn't
+ * need the RSB filling sequence. But it does need to be enabled
+ * before the first unbalanced RET.
+ *
+ * So no RETs before vmx_spec_ctrl_restore_host() below.
+ */
+ vmexit_fill_RSB();
+
+ /* Save this for below */
+ spec_ctrl = vmx_spec_ctrl_restore_host(vmx);
vmx_enable_fb_clear(vmx);
@@ -9986,12 +10024,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
* save it.
*/
if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
- vmx->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
-
- x86_spec_ctrl_restore_host(vmx->spec_ctrl, 0);
-
- /* Eliminate branch target predictions from guest mode */
- vmexit_fill_RSB();
+ vmx->spec_ctrl = spec_ctrl;
/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
if (debugctlmsr)
--
2.17.1
