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xen.28173
xsa404-1.patch
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File xsa404-1.patch of Package xen.28173
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> --- a/docs/misc/xen-command-line.pandoc +++ b/docs/misc/xen-command-line.pandoc @@ -1976,9 +1976,8 @@ 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 @@ -605,6 +605,8 @@ int arch_domain_create(struct domain *d, */ d->arch.x87_fip_width = cpu_has_fpu_sel ? 0 : 8; + spec_ctrl_init_domain(d); + return 0; fail: @@ -1691,13 +1693,14 @@ 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 = next->dirty_cpu; 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 ) { @@ -1763,6 +1766,11 @@ 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; } context_saved(prev); --- a/xen/arch/x86/hvm/vmx/entry.S +++ b/xen/arch/x86/hvm/vmx/entry.S @@ -86,6 +86,7 @@ UNLIKELY_END(realmode) /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */ SPEC_CTRL_EXIT_TO_HVM /* Req: a=spec_ctrl %rsp=regs/cpuinfo, Clob: cd */ + 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 @@ -35,8 +35,8 @@ 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 { @@ -896,6 +896,13 @@ 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; @@ -1093,21 +1100,20 @@ 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 @@ -33,8 +33,7 @@ XEN_CPUFEATURE(SC_RSB_HVM, (FSCAPIN XEN_CPUFEATURE(XEN_SELFSNOOP, (FSCAPINTS+0)*32+20) /* SELFSNOOP gets used by Xen itself */ XEN_CPUFEATURE(SC_MSR_IDLE, (FSCAPINTS+0)*32+21) /* (SC_MSR_PV || SC_MSR_HVM) && default_xen_spec_ctrl */ XEN_CPUFEATURE(XEN_LBR, (FSCAPINTS+0)*32+22) /* Xen uses MSR_DEBUGCTL.LBR */ -XEN_CPUFEATURE(SC_VERW_PV, (FSCAPINTS+0)*32+23) /* VERW used by Xen for PV */ -XEN_CPUFEATURE(SC_VERW_HVM, (FSCAPINTS+0)*32+24) /* VERW used by Xen for HVM */ +/* Bits 23,24 unused. */ XEN_CPUFEATURE(SC_VERW_IDLE, (FSCAPINTS+0)*32+25) /* VERW used by Xen for idle */ /* Bug words follow the synthetic words. */ --- a/xen/include/asm-x86/domain.h +++ b/xen/include/asm-x86/domain.h @@ -296,6 +296,9 @@ struct arch_domain uint32_t pci_cf8; uint8_t cmos_idx; + /* Use VERW on return-to-guest for its flushing side effect. */ + bool verw; + bool_t s3_integrity; struct list_head pdev_list; --- 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 @@ -141,6 +141,19 @@ wrmsr .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) @@ -242,15 +255,12 @@ #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 when exiting to HVM guest context. */ #define SPEC_CTRL_EXIT_TO_HVM \ ALTERNATIVE "", \ DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_HVM; \ - ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)), \ - X86_FEATURE_SC_VERW_HVM /* * Use in IST interrupt/exception context. May interrupt Xen or PV context.
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