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xen.28171
62ab0fab-x86-spec-ctrl-VERW-flushing-runtime-co...
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File 62ab0fab-x86-spec-ctrl-VERW-flushing-runtime-cond.patch of Package xen.28171
# Commit e06b95c1d44ab80da255219fc9f1e2fc423edcb6 # Date 2022-06-16 12:10:37 +0100 # Author Andrew Cooper <andrew.cooper3@citrix.com> # Committer Andrew Cooper <andrew.cooper3@citrix.com> 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> --- a/docs/misc/xen-command-line.pandoc +++ b/docs/misc/xen-command-line.pandoc @@ -2129,9 +2129,8 @@ 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. --- a/xen/arch/x86/domain.c +++ b/xen/arch/x86/domain.c @@ -651,6 +651,8 @@ int arch_domain_create(struct domain *d, domain_cpu_policy_changed(d); + spec_ctrl_init_domain(d); + return 0; fail: @@ -1763,14 +1765,15 @@ 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 ) { @@ -1834,6 +1837,11 @@ void context_switch(struct vcpu *prev, s *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); --- 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 --- 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 = t static bool __initdata opt_msr_sc_hvm = true; static bool __initdata opt_rsb_pv = true; 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 { @@ -903,6 +903,13 @@ static __init void mds_calculations(uint } } +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; @@ -1148,21 +1155,20 @@ void __init init_speculation_mitigations 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 --- a/xen/include/asm-x86/cpufeatures.h +++ b/xen/include/asm-x86/cpufeatures.h @@ -35,8 +35,7 @@ XEN_CPUFEATURE(SC_RSB_HVM, X86_SY 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 */ --- a/xen/include/asm-x86/domain.h +++ b/xen/include/asm-x86/domain.h @@ -308,6 +308,9 @@ 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; --- 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; --- 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.
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