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In-Reply-To: <20191206102649.GC2844@hirez.programming.kicks-ass.net>
From:   Thomas Garnier <thgarnie@chromium.org>
Date:   Fri, 6 Dec 2019 08:35:09 -0800
Message-ID: <CAJcbSZHLcSN4BK=N7M3Kv9q-hkPe6dDxbHaRCG9v2JVwhSZxfw@mail.gmail.com>
Subject: Re: [PATCH v10 04/11] x86/entry/64: Adapt assembly for PIE support
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Fri, Dec 6, 2019 at 2:27 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Dec 05, 2019 at 09:01:50AM -0800, Thomas Garnier wrote:
> > On Thu, Dec 5, 2019 at 1:04 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > On Wed, Dec 04, 2019 at 04:09:41PM -0800, Thomas Garnier wrote:
> > >
> > > > @@ -1625,7 +1627,11 @@ first_nmi:
> > > >       addq    $8, (%rsp)      /* Fix up RSP */
> > > >       pushfq                  /* RFLAGS */
> > > >       pushq   $__KERNEL_CS    /* CS */
> > > > -     pushq   $1f             /* RIP */
> > > > +     pushq   $0              /* Future return address */
> > >
> > > We're building an IRET frame, the IRET frame does not have a 'future
> > > return address' field.
> >
> > I assumed that's the target RIP after iretq.
>
> It is. But it's still the (R)IP field of the IRET frame. Calling it
> anything else is just confusing. The frame is 5 words: SS, (R)SP, (R)FLAGS,
> CS, (R)IP.
>
> > > > +     pushq   %rdx            /* Save RAX */
> > > > +     leaq    1f(%rip), %rdx  /* RIP */
> > >
> > > nonsensical comment
> >
> > That was the same comment from the push $1f that I changed.
>
> Yes, but there it made sense since the PUSH actually created that field
> of the frame, here it is nonsensical. What this instruction does is put
> the address of the '1f' label into RDX, which is then stuck into the
> (R)IP field on the next instruction.

Got it, make sense. Thanks.

>
> > > > +     movq    %rdx, 8(%rsp)   /* Put 1f on return address */
> > > > +     popq    %rdx            /* Restore RAX */