From: Sean Christopherson sean.j.christopherson@intel.com
commit 75ee23b30dc712d80d2421a9a547e7ab6e379b44 upstream.
Don't advance RIP or inject a single-step #DB if emulation signals a fault. This logic applies to all state updates that are conditional on clean retirement of the emulation instruction, e.g. updating RFLAGS was previously handled by commit 38827dbd3fb85 ("KVM: x86: Do not update EFLAGS on faulting emulation").
Not advancing RIP is likely a nop, i.e. ctxt->eip isn't updated with ctxt->_eip until emulation "retires" anyways. Skipping #DB injection fixes a bug reported by Andy Lutomirski where a #UD on SYSCALL due to invalid state with EFLAGS.TF=1 would loop indefinitely due to emulation overwriting the #UD with #DB and thus restarting the bad SYSCALL over and over.
Cc: Nadav Amit nadav.amit@gmail.com Cc: stable@vger.kernel.org Reported-by: Andy Lutomirski luto@kernel.org Fixes: 663f4c61b803 ("KVM: x86: handle singlestep during emulation") Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Radim Krčmář rkrcmar@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/kvm/x86.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)
--- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5823,12 +5823,13 @@ restart: unsigned long rflags = kvm_x86_ops->get_rflags(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 && ctxt->tf) - kvm_vcpu_do_singlestep(vcpu, &r); if (!ctxt->have_exception || - exception_type(ctxt->exception.vector) == EXCPT_TRAP) + exception_type(ctxt->exception.vector) == EXCPT_TRAP) { + kvm_rip_write(vcpu, ctxt->eip); + if (r == EMULATE_DONE && ctxt->tf) + kvm_vcpu_do_singlestep(vcpu, &r); __kvm_set_rflags(vcpu, ctxt->eflags); + }
/* * For STI, interrupts are shadowed; so KVM_REQ_EVENT will