From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: x86/spec-ctrl: Make VERW flushing runtime conditional
Currently, VERW flushing to mitigate MDS is boot time conditional per domain
type. However, to provide mitigations for DRPW (CVE-2022-21166), we need to
conditionally use VERW based on the trustworthiness of the guest, and the
devices passed through.
Remove the PV/HVM alternatives and instead issue a VERW on the return-to-guest
path depending on the SCF_verw bit in cpuinfo spec_ctrl_flags.
Introduce spec_ctrl_init_domain() and d->arch.verw to calculate the VERW
disposition at domain creation time, and context switch the SCF_verw bit.
For now, VERW flushing is used and controlled exactly as before, but later
patches will add per-domain cases too.
No change in behaviour.
This is part of XSA-404.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 1d08fb7e9aa6..d5cb09f86541 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2258,9 +2258,8 @@ in place for guests to use.
Use of a positive boolean value for either of these options is invalid.
The booleans `pv=`, `hvm=`, `msr-sc=`, `rsb=` and `md-clear=` offer fine
-grained control over the alternative blocks used by Xen. These impact Xen's
-ability to protect itself, and Xen's ability to virtualise support for guests
-to use.
+grained control over the primitives by Xen. These impact Xen's ability to
+protect itself, and Xen's ability to virtualise support for guests to use.
* `pv=` and `hvm=` offer control over all suboptions for PV and HVM guests
respectively.
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index ef1812dc1402..1fe6644a71ae 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -863,6 +863,8 @@ int arch_domain_create(struct domain *d,
d->arch.msr_relaxed = config->arch.misc_flags & XEN_X86_MSR_RELAXED;
+ spec_ctrl_init_domain(d);
+
return 0;
fail:
@@ -2017,14 +2019,15 @@ static void __context_switch(void)
void context_switch(struct vcpu *prev, struct vcpu *next)
{
unsigned int cpu = smp_processor_id();
+ struct cpu_info *info = get_cpu_info();
const struct domain *prevd = prev->domain, *nextd = next->domain;
unsigned int dirty_cpu = read_atomic(&next->dirty_cpu);
ASSERT(prev != next);
ASSERT(local_irq_is_enabled());
- get_cpu_info()->use_pv_cr3 = false;
- get_cpu_info()->xen_cr3 = 0;
+ info->use_pv_cr3 = false;
+ info->xen_cr3 = 0;
if ( unlikely(dirty_cpu != cpu) && dirty_cpu != VCPU_CPU_CLEAN )
{
@@ -2088,6 +2091,11 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
*last_id = next_id;
}
}
+
+ /* Update the top-of-stack block with the VERW disposition. */
+ info->spec_ctrl_flags &= ~SCF_verw;
+ if ( nextd->arch.verw )
+ info->spec_ctrl_flags |= SCF_verw;
}
sched_context_switched(prev, next);
diff --git a/xen/arch/x86/hvm/vmx/entry.S b/xen/arch/x86/hvm/vmx/entry.S
index 49651f3c435a..5f5de45a1309 100644
--- a/xen/arch/x86/hvm/vmx/entry.S
+++ b/xen/arch/x86/hvm/vmx/entry.S
@@ -87,7 +87,7 @@ UNLIKELY_END(realmode)
/* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
/* SPEC_CTRL_EXIT_TO_VMX Req: %rsp=regs/cpuinfo Clob: */
- ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)), X86_FEATURE_SC_VERW_HVM
+ DO_SPEC_CTRL_COND_VERW
mov VCPU_hvm_guest_cr2(%rbx),%rax
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index c19464da70ce..21730aa03071 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -36,8 +36,8 @@ static bool __initdata opt_msr_sc_pv = true;
static bool __initdata opt_msr_sc_hvm = true;
static int8_t __initdata opt_rsb_pv = -1;
static bool __initdata opt_rsb_hvm = true;
-static int8_t __initdata opt_md_clear_pv = -1;
-static int8_t __initdata opt_md_clear_hvm = -1;
+static int8_t __read_mostly opt_md_clear_pv = -1;
+static int8_t __read_mostly opt_md_clear_hvm = -1;
/* Cmdline controls for Xen's speculative settings. */
static enum ind_thunk {
@@ -932,6 +932,13 @@ static __init void mds_calculations(uint64_t caps)
}
}
+void spec_ctrl_init_domain(struct domain *d)
+{
+ bool pv = is_pv_domain(d);
+
+ d->arch.verw = pv ? opt_md_clear_pv : opt_md_clear_hvm;
+}
+
void __init init_speculation_mitigations(void)
{
enum ind_thunk thunk = THUNK_DEFAULT;
@@ -1196,21 +1203,20 @@ void __init init_speculation_mitigations(void)
boot_cpu_has(X86_FEATURE_MD_CLEAR));
/*
- * Enable MDS defences as applicable. The PV blocks need using all the
- * time, and the Idle blocks need using if either PV or HVM defences are
- * used.
+ * Enable MDS defences as applicable. The Idle blocks need using if
+ * either PV or HVM defences are used.
*
* HVM is more complicated. The MD_CLEAR microcode extends L1D_FLUSH with
- * equivelent semantics to avoid needing to perform both flushes on the
- * HVM path. The HVM blocks don't need activating if our hypervisor told
- * us it was handling L1D_FLUSH, or we are using L1D_FLUSH ourselves.
+ * equivalent semantics to avoid needing to perform both flushes on the
+ * HVM path. Therefore, we don't need VERW in addition to L1D_FLUSH.
+ *
+ * After calculating the appropriate idle setting, simplify
+ * opt_md_clear_hvm to mean just "should we VERW on the way into HVM
+ * guests", so spec_ctrl_init_domain() can calculate suitable settings.
*/
- if ( opt_md_clear_pv )
- setup_force_cpu_cap(X86_FEATURE_SC_VERW_PV);
if ( opt_md_clear_pv || opt_md_clear_hvm )
setup_force_cpu_cap(X86_FEATURE_SC_VERW_IDLE);
- if ( opt_md_clear_hvm && !(caps & ARCH_CAPS_SKIP_L1DFL) && !opt_l1d_flush )
- setup_force_cpu_cap(X86_FEATURE_SC_VERW_HVM);
+ opt_md_clear_hvm &= !(caps & ARCH_CAPS_SKIP_L1DFL) && !opt_l1d_flush;
/*
* Warn the user if they are on MLPDS/MFBDS-vulnerable hardware with HT
diff --git a/xen/include/asm-x86/cpufeatures.h b/xen/include/asm-x86/cpufeatures.h
index ff3157d52d13..bd45a144ee78 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -35,8 +35,7 @@ XEN_CPUFEATURE(SC_RSB_HVM, X86_SYNTH(19)) /* RSB overwrite needed for HVM
XEN_CPUFEATURE(XEN_SELFSNOOP, X86_SYNTH(20)) /* SELFSNOOP gets used by Xen itself */
XEN_CPUFEATURE(SC_MSR_IDLE, X86_SYNTH(21)) /* (SC_MSR_PV || SC_MSR_HVM) && default_xen_spec_ctrl */
XEN_CPUFEATURE(XEN_LBR, X86_SYNTH(22)) /* Xen uses MSR_DEBUGCTL.LBR */
-XEN_CPUFEATURE(SC_VERW_PV, X86_SYNTH(23)) /* VERW used by Xen for PV */
-XEN_CPUFEATURE(SC_VERW_HVM, X86_SYNTH(24)) /* VERW used by Xen for HVM */
+/* Bits 23,24 unused. */
XEN_CPUFEATURE(SC_VERW_IDLE, X86_SYNTH(25)) /* VERW used by Xen for idle */
XEN_CPUFEATURE(XEN_SHSTK, X86_SYNTH(26)) /* Xen uses CET Shadow Stacks */
XEN_CPUFEATURE(XEN_IBT, X86_SYNTH(27)) /* Xen uses CET Indirect Branch Tracking */
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 92d54de0b9a1..2398a1d99da9 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -319,6 +319,9 @@ struct arch_domain
uint32_t pci_cf8;
uint8_t cmos_idx;
+ /* Use VERW on return-to-guest for its flushing side effect. */
+ bool verw;
+
union {
struct pv_domain pv;
struct hvm_domain hvm;
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index f76029523610..751355f471f4 100644
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -24,6 +24,7 @@
#define SCF_use_shadow (1 << 0)
#define SCF_ist_wrmsr (1 << 1)
#define SCF_ist_rsb (1 << 2)
+#define SCF_verw (1 << 3)
#ifndef __ASSEMBLY__
@@ -32,6 +33,7 @@
#include <asm/msr-index.h>
void init_speculation_mitigations(void);
+void spec_ctrl_init_domain(struct domain *d);
extern bool opt_ibpb;
extern bool opt_ssbd;
diff --git a/xen/include/asm-x86/spec_ctrl_asm.h b/xen/include/asm-x86/spec_ctrl_asm.h
index 02b3b18ce69f..5a590bac44aa 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -136,6 +136,19 @@
#endif
.endm
+.macro DO_SPEC_CTRL_COND_VERW
+/*
+ * Requires %rsp=cpuinfo
+ *
+ * Issue a VERW for its flushing side effect, if indicated. This is a Spectre
+ * v1 gadget, but the IRET/VMEntry is serialising.
+ */
+ testb $SCF_verw, CPUINFO_spec_ctrl_flags(%rsp)
+ jz .L\@_verw_skip
+ verw CPUINFO_verw_sel(%rsp)
+.L\@_verw_skip:
+.endm
+
.macro DO_SPEC_CTRL_ENTRY maybexen:req
/*
* Requires %rsp=regs (also cpuinfo if !maybexen)
@@ -231,8 +244,7 @@
#define SPEC_CTRL_EXIT_TO_PV \
ALTERNATIVE "", \
DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_PV; \
- ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)), \
- X86_FEATURE_SC_VERW_PV
+ DO_SPEC_CTRL_COND_VERW
/*
* Use in IST interrupt/exception context. May interrupt Xen or PV context.