From: Jim Mattson <jmattson@google.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Radim Krčmář" <rkrcmar@redhat.com>,
"Vitaly Kuznetsov" <vkuznets@redhat.com>,
"Wanpeng Li" <wanpengli@tencent.com>,
"Joerg Roedel" <joro@8bytes.org>,
"kvm list" <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"Nadav Amit" <nadav.amit@gmail.com>,
"Andy Lutomirski" <luto@kernel.org>
Subject: Re: [PATCH] KVM: x86: Don't update RIP or do single-step on faulting emulation
Date: Tue, 27 Aug 2019 12:12:51 -0700 [thread overview]
Message-ID: <CALMp9eSwxTdigRkACRgr=avg8HZh+gPXgPnwd7+CaNEEuS2tQA@mail.gmail.com> (raw)
In-Reply-To: <20190823205544.24052-1-sean.j.christopherson@intel.com>
On Fri, Aug 23, 2019 at 1:55 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> 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 RFLAGS.RF=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>
> ---
>
> Note, this has minor conflict with my recent series to cleanup the
> emulator return flows[*]. The end result should look something like:
>
> if (!ctxt->have_exception ||
> exception_type(ctxt->exception.vector) == EXCPT_TRAP) {
> kvm_rip_write(vcpu, ctxt->eip);
> if (r && ctxt->tf)
> r = kvm_vcpu_do_singlestep(vcpu);
> __kvm_set_rflags(vcpu, ctxt->eflags);
> }
>
> [*] https://lkml.kernel.org/r/20190823010709.24879-1-sean.j.christopherson@intel.com
>
> arch/x86/kvm/x86.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b4cfd786d0b6..d2962671c3d3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6611,12 +6611,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> 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) {
NYC, but...
I don't think this check for "exception_type" is quite right. A
general detect fault (which can be synthesized by check_dr_read) is
mischaracterized by exception_type() as a trap. Or maybe I'm missing
something? (I often am.)
> + 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
> --
> 2.22.0
>
next prev parent reply other threads:[~2019-08-27 19:13 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-23 20:55 [PATCH] KVM: x86: Don't update RIP or do single-step on faulting emulation Sean Christopherson
2019-08-23 21:42 ` Nadav Amit
2019-08-23 22:46 ` Andy Lutomirski
2019-08-26 14:06 ` Sean Christopherson
2019-08-27 18:35 ` Radim Krčmář
2019-08-27 19:12 ` Jim Mattson [this message]
2019-08-27 19:49 ` Sean Christopherson
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