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From:   Jann Horn <jannh@google.com>
Date:   Wed, 18 Dec 2019 23:33:54 +0100
Message-ID: <CAG48ez1-u8DbxSAu9DXTEEy3-ADquQLWXB6ufV+By7TnuxWOsQ@mail.gmail.com>
Subject: Re: [PATCH v6 4/4] x86/kasan: Print original address on #GP
To:     Borislav Petkov <bp@alien8.de>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>
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On Wed, Dec 11, 2019 at 6:37 PM Borislav Petkov <bp@alien8.de> wrote:
> On Mon, Dec 09, 2019 at 03:31:20PM +0100, Jann Horn wrote:
> >  arch/x86/kernel/traps.c     | 12 ++++++++++-
> >  arch/x86/mm/kasan_init_64.c | 21 -------------------
> >  include/linux/kasan.h       |  6 ++++++
> >  mm/kasan/report.c           | 40 +++++++++++++++++++++++++++++++++++++
> >  4 files changed, 57 insertions(+), 22 deletions(-)
>
> I need a KASAN person ACK here, I'd guess.

Right, I got a Reviewed-by from Dmitry for v2, but cleared that when I
made changes to the patch later - I'll ask Dmitry for a fresh ack on
the v7 patch.

[...]
> > -             die(desc, regs, error_code);
> > +             flags = oops_begin();
> > +             sig = SIGSEGV;
> > +             __die_header(desc, regs, error_code);
> > +             if (hint == GP_NON_CANONICAL)
> > +                     kasan_non_canonical_hook(gp_addr);
> > +             if (__die_body(desc, regs, error_code))
> > +                     sig = 0;
> > +             oops_end(flags, regs, sig);
>
> Instead of opencoding it like this, can we add a
>
>         die_addr(desc, regs, error_code, gp_addr);
>
> to arch/x86/kernel/dumpstack.c and call it from here:
>
>         if (hint != GP_NON_CANONICAL)
>                 gp_addr = 0;
>
>         die_addr(desc, regs, error_code, gp_addr);

Okay, so I'll make __die_header() and __die_body() static, introduce
and hook up die_addr() in patch 3/4, and then in patch 4/4 insert the
call to the KASAN hook.

> This way you won't need to pass down to die_addr() the hint too - you
> code into gp_addr whether it was non-canonical or not.
>
> The
>
> +       if (addr < KASAN_SHADOW_OFFSET)
> +               return;
>
> check in kasan_non_canonical_hook() would then catch it when addr == 0.

I'll add an explicit check for nonzero address before calling
kasan_non_canonical_hook() so that the semantics are a bit more
cleanly split.