linux-kernel - Re: [PATCH] KVM: x86: Don't update RIP or do single-step on faulting emulation

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Message-Id: <D49D02CD-4D29-48C0-B61C-FBE50C79D802@gmail.com>
Date:   Fri, 23 Aug 2019 14:42:46 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] KVM: x86: Don't update RIP or do single-step on faulting
 emulation

> On Aug 23, 2019, at 1:55 PM, Sean Christopherson <sean.j.christopherson@...el.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@...il.com>
> Cc: stable@...r.kernel.org
> Reported-by: Andy Lutomirski <luto@...nel.org>
> Fixes: 663f4c61b803 ("KVM: x86: handle singlestep during emulation")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>

Seems fine. I guess I should’ve found it before…

Consider running the relevant self-tests (e.g., single_test_syscall) to
avoid regressions.