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SubjectRe: [PATCH for 4.4-stable] KVM: x86: fix singlestepping over syscall
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On 16.11.2017 19:07, Paolo Bonzini wrote:
> [ Upstream commit c8401dda2f0a00cd25c0af6a95ed50e478d25de4 ]
>
> TF is handled a bit differently for syscall and sysret, compared
> to the other instructions: TF is checked after the instruction completes,
> so that the OS can disable #DB at a syscall by adding TF to FMASK.
> When the sysret is executed the #DB is taken "as if" the syscall insn
> just completed.
>
> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
> Fix the behavior, otherwise you could get #DB on a user stack which is not
> nice. This does not affect Linux guests, as they use an IST or task gate
> for #DB.
>
> This fixes CVE-2017-7518.
>
> Cc: stable@vger.kernel.org
> Reported-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
>
> Conflicts:
> arch/x86/kvm/x86.c
> ---
> arch/x86/include/asm/kvm_emulate.h | 1 +
> arch/x86/kvm/emulate.c | 1 +
> arch/x86/kvm/x86.c | 52 ++++++++++++++++----------------------
> 3 files changed, 24 insertions(+), 30 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index 19d14ac23ef9..fc3c7e49c8e4 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -296,6 +296,7 @@ struct x86_emulate_ctxt {
>
> bool perm_ok; /* do not check permissions if true */
> bool ud; /* inject an #UD if host doesn't support insn */
> + bool tf; /* TF value before instruction (after for syscall/sysret) */
>
> bool have_exception;
> struct x86_exception exception;
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 04b2f3cad7ba..684edebb4a0c 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2726,6 +2726,7 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
> ctxt->eflags &= ~(X86_EFLAGS_VM | X86_EFLAGS_IF);
> }
>
> + ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
> return X86EMUL_CONTINUE;
> }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8e526c6fd784..3ffd5900da5b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5095,6 +5095,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
> kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>
> ctxt->eflags = kvm_get_rflags(vcpu);
> + ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
> +
> ctxt->eip = kvm_rip_read(vcpu);
> ctxt->mode = (!is_protmode(vcpu)) ? X86EMUL_MODE_REAL :
> (ctxt->eflags & X86_EFLAGS_VM) ? X86EMUL_MODE_VM86 :
> @@ -5315,37 +5317,26 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
> return dr6;
> }
>
> -static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflags, int *r)
> +static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
> {
> struct kvm_run *kvm_run = vcpu->run;
>
> - /*
> - * rflags is the old, "raw" value of the flags. The new value has
> - * not been saved yet.
> - *
> - * This is correct even for TF set by the guest, because "the
> - * processor will not generate this exception after the instruction
> - * that sets the TF flag".
> - */
> - if (unlikely(rflags & X86_EFLAGS_TF)) {
> - if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> - kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
> - DR6_RTM;
> - kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
> - kvm_run->debug.arch.exception = DB_VECTOR;
> - kvm_run->exit_reason = KVM_EXIT_DEBUG;
> - *r = EMULATE_USER_EXIT;
> - } else {
> - vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF;
> - /*
> - * "Certain debug exceptions may clear bit 0-3. The
> - * remaining contents of the DR6 register are never
> - * cleared by the processor".
> - */
> - vcpu->arch.dr6 &= ~15;
> - vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> - kvm_queue_exception(vcpu, DB_VECTOR);
> - }
> + if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> + kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM;
> + kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
> + kvm_run->debug.arch.exception = DB_VECTOR;
> + kvm_run->exit_reason = KVM_EXIT_DEBUG;
> + *r = EMULATE_USER_EXIT;
> + } else {
> + vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF;
> + /*
> + * "Certain debug exceptions may clear bit 0-3. The
> + * remaining contents of the DR6 register are never
> + * cleared by the processor".
> + */
> + vcpu->arch.dr6 &= ~15;
> + vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> + kvm_queue_exception(vcpu, DB_VECTOR);
> }
> }

This looks good to me.

General question: how do we treat KVM single stepping and concurrent TF
in the guest? IOW, shouldn't the "else" rather be a check for ctxt->tf ?
(or is that handled later on e.g. in user space?)

But question unrelated to this patch, so

Reviewed-by: David Hildenbrand <david@redhat.com>

>
> @@ -5500,8 +5491,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> toggle_interruptibility(vcpu, ctxt->interruptibility);
> vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
> kvm_rip_write(vcpu, ctxt->eip);
> - if (r == EMULATE_DONE)
> - kvm_vcpu_check_singlestep(vcpu, rflags, &r);
> + if (r == EMULATE_DONE &&
> + (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
> + kvm_vcpu_do_singlestep(vcpu, &r);
> if (!ctxt->have_exception ||
> exception_type(ctxt->exception.vector) == EXCPT_TRAP)
> __kvm_set_rflags(vcpu, ctxt->eflags);
>


--

Thanks,

David / dhildenb

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