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KVM: x86: fix singlestepping over syscall 

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>
This commit is contained in:
Paolo Bonzini 2017-06-07 15:13:14 +02:00 committed by Radim Krčmář
parent d6aa07c169
commit c8401dda2f
3 changed files with 34 additions and 30 deletions
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1
arch/x86/include/asm/kvm_emulate.h
View File

@ -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;

1
arch/x86/kvm/emulate.c
View File

@ -2742,6 +2742,7 @@ 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;
}

62
arch/x86/kvm/x86.c
View File

@ -5313,6 +5313,8 @@ 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 :
@ -5528,36 +5530,25 @@ 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 {
/*
* "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 {
/*
* "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);
}
}
@ -5567,7 +5558,17 @@ int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
int r = EMULATE_DONE;
kvm_x86_ops->skip_emulated_instruction(vcpu);
kvm_vcpu_check_singlestep(vcpu, rflags, &r);
/*
* 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))
kvm_vcpu_do_singlestep(vcpu, &r);
return r == EMULATE_DONE;
}
EXPORT_SYMBOL_GPL(kvm_skip_emulated_instruction);
@ -5726,8 +5727,9 @@ restart:
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);