Messages in this thread Patch in this message | | | Date | Thu, 2 Apr 2020 08:30:35 -0700 | From | Sean Christopherson <> | Subject | Re: [patch 2/2] x86/kvm/vmx: Prevent split lock detection induced #AC wreckage |
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On Thu, Apr 02, 2020 at 02:33:00PM +0200, Thomas Gleixner wrote: > Without at least minimal handling for split lock detection induced #AC, VMX > will just run into the same problem as the VMWare hypervisor, which was > reported by Kenneth. > > It will inject the #AC blindly into the guest whether the guest is prepared > or not. > > Add the minimal required handling for it: > > - Check guest state whether CR0.AM is enabled and EFLAGS.AC is set. If > so, then the #AC originated from CPL3 and the guest has is prepared to > handle it. In this case it does not matter whether the #AC is due to a > split lock or a regular unaligned check. > > - Invoke a minimal split lock detection handler. If the host SLD mode is > sld_warn, then handle it in the same way as user space handling works: > Emit a warning, disable SLD and mark the current task with TIF_SLD. > With that resume the guest without injecting #AC. > > If the host mode is sld_fatal or sld_off, emit a warning and deliver > the exception to user space which can crash and burn itself. > > Mark the module with MOD_INFO(sld_safe, "Y") so the module loader does not > force SLD off.
Some comments below. But, any objection to taking Xiaoyao's patches that do effectively the same things, minus the MOD_INFO()? I'll repost them in reply to this thread.
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > Cc: "Kenneth R. Crudup" <kenny@panix.com> > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Fenghua Yu <fenghua.yu@intel.com> > Cc: Xiaoyao Li <xiaoyao.li@intel.com> > Cc: Nadav Amit <namit@vmware.com> > Cc: Thomas Hellstrom <thellstrom@vmware.com> > Cc: Sean Christopherson <sean.j.christopherson@intel.com> > Cc: Tony Luck <tony.luck@intel.com> > --- > arch/x86/include/asm/cpu.h | 1 + > arch/x86/kernel/cpu/intel.c | 28 +++++++++++++++++++++++----- > arch/x86/kvm/vmx/vmx.c | 40 +++++++++++++++++++++++++++++++++++++--- > 3 files changed, 61 insertions(+), 8 deletions(-) > > --- a/arch/x86/include/asm/cpu.h > +++ b/arch/x86/include/asm/cpu.h > @@ -44,6 +44,7 @@ unsigned int x86_stepping(unsigned int s > extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c); > extern void switch_to_sld(unsigned long tifn); > extern bool handle_user_split_lock(struct pt_regs *regs, long error_code); > +extern bool handle_guest_split_lock(unsigned long ip); > extern void split_lock_validate_module_text(struct module *me, void *text, void *text_end); > #else > static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {} > --- a/arch/x86/kernel/cpu/intel.c > +++ b/arch/x86/kernel/cpu/intel.c > @@ -1102,13 +1102,10 @@ static void split_lock_init(void) > split_lock_verify_msr(sld_state != sld_off); > } > > -bool handle_user_split_lock(struct pt_regs *regs, long error_code) > +static void split_lock_warn(unsigned long ip) > { > - if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal) > - return false; > - > pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n", > - current->comm, current->pid, regs->ip); > + current->comm, current->pid, ip); > > /* > * Disable the split lock detection for this task so it can make > @@ -1117,6 +1114,27 @@ bool handle_user_split_lock(struct pt_re > */ > sld_update_msr(false); > set_tsk_thread_flag(current, TIF_SLD); > +} > + > +bool handle_guest_split_lock(unsigned long ip) > +{ > + if (sld_state == sld_warn) { > + split_lock_warn(ip); > + return true; > + } > + > + pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n", > + current->comm, current->pid, > + sld_state == sld_fatal ? "fatal" : "bogus", ip); > + return false; > +} > +EXPORT_SYMBOL_GPL(handle_guest_split_lock); > + > +bool handle_user_split_lock(struct pt_regs *regs, long error_code) > +{ > + if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal) > + return false; > + split_lock_warn(regs->ip); > return true; > } > > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -65,6 +65,7 @@ > > MODULE_AUTHOR("Qumranet"); > MODULE_LICENSE("GPL"); > +MODULE_INFO(sld_safe, "Y"); > > #ifdef MODULE > static const struct x86_cpu_id vmx_cpu_id[] = { > @@ -4623,6 +4624,22 @@ static int handle_machine_check(struct k > return 1; > } > > +static bool guest_handles_ac(struct kvm_vcpu *vcpu) > +{ > + /* > + * If guest has alignment checking enabled in CR0 and activated in > + * eflags, then the #AC originated from CPL3 and the guest is able > + * to handle it. It does not matter whether this is a regular or > + * a split lock operation induced #AC. > + */ > + if (vcpu->arch.cr0 & X86_CR0_AM &&
Technically not required since KVM doesn't let the gets toggle CR0.AM at will, but going through kvm_read_cr0{_bits}() is preferred.
> + vmx_get_rflags(vcpu) & X86_EFLAGS_AC)
I don't think this is correct. A guest could trigger a split-lock #AC at CPL0 with EFLAGS.AC=1 and CR0.AM=1, and then panic because it didn't expect #AC at CPL0.
> + return true; > + > + /* Add guest SLD handling checks here once it's supported */ > + return false; > +} > + > static int handle_exception_nmi(struct kvm_vcpu *vcpu) > { > struct vcpu_vmx *vmx = to_vmx(vcpu); > @@ -4688,9 +4705,6 @@ static int handle_exception_nmi(struct k > return handle_rmode_exception(vcpu, ex_no, error_code); > > switch (ex_no) { > - case AC_VECTOR: > - kvm_queue_exception_e(vcpu, AC_VECTOR, error_code); > - return 1; > case DB_VECTOR: > dr6 = vmcs_readl(EXIT_QUALIFICATION); > if (!(vcpu->guest_debug & > @@ -4719,6 +4733,26 @@ static int handle_exception_nmi(struct k > kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip; > kvm_run->debug.arch.exception = ex_no; > break; > + case AC_VECTOR: > + if (guest_handles_ac(vcpu)) { > + kvm_queue_exception_e(vcpu, AC_VECTOR, error_code); > + return 1; > + } > + /* > + * Handle #AC caused by split lock detection. If the host > + * mode is sld_warn, then it warns, marks current with > + * TIF_SLD and disables split lock detection. So the guest > + * can just continue. > + * > + * If the host mode is fatal, the handling code warned. Let > + * qemu kill itself. > + * > + * If the host mode is off, then this #AC is bonkers and > + * something is badly wrong. Let it fail as well. > + */ > + if (handle_guest_split_lock(kvm_rip_read(vcpu))) > + return 1; > + /* fall through */ > default: > kvm_run->exit_reason = KVM_EXIT_EXCEPTION; > kvm_run->ex.exception = ex_no; >
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