linux-kernel - Re: KASAN vs. boot-time switching between 4- and 5-level paging

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Date:   Tue, 11 Jul 2017 08:06:57 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: KASAN vs. boot-time switching between 4- and 5-level paging

On Tue, Jul 11, 2017 at 3:35 AM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
> On Mon, Jul 10, 2017 at 05:30:38PM -0700, Andy Lutomirski wrote:
>> On Mon, Jul 10, 2017 at 2:24 PM, Kirill A. Shutemov
>> <kirill@...temov.name> wrote:
>> > On Mon, Jul 10, 2017 at 01:07:13PM -0700, Andy Lutomirski wrote:
>> >> Can you give the disassembly of the backtrace lines?  Blaming the
>> >> .endr doesn't make much sense to me.
>> >
>> > I don't have backtrace. It's before printk() is functional. I only see
>> > triple fault and reboot.
>> >
>> > I had to rely on qemu tracing and gdb.
>>
>> Can you ask GDB or objtool to disassemble around those addresses?  Can
>> you also attach the big dump that QEMU throws out that shows register
>> state?  In particular, CR2, CR3, and CR4 could be useful.
>
> The last three execptions:
>
> check_exception old: 0xffffffff new 0xe, cr2: 0xffffffff7ffffff8, rip: 0xffffffff84bb3036
> RAX=00000000ffffffff RBX=ffffffff800000d8 RCX=ffffffff84be4021 RDX=dffffc0000000000
> RSI=0000000000000006 RDI=ffffffff84c57000 RBP=ffffffff800000c8 RSP=ffffffff80000000

So RSP was 0xffffffff80000000, a push happened, and we tried to write
to 0xffffffff7ffffff8, which failed.

> check_exception old: 0xe new 0xe, cr2: 0xffffffff7ffffff8, rip: 0xffffffff84bb3141
> RAX=00000000ffffffff RBX=ffffffff800000d8 RCX=ffffffff84be4021 RDX=dffffc0000000000
> RSI=0000000000000006 RDI=ffffffff84c57000 RBP=ffffffff800000c8 RSP=ffffffff80000000

And #PF doesn't use IST, so it double-faulted.

Either the stack isn't mapped in the page tables, RSP is corrupt, or
there's a genuine stack overflow here.