linux-kernel - Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

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Date:   Fri, 5 Jan 2018 14:48:16 -0800
From:   Hugh Dickins <hughd@...gle.com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, richard.fellner@...dent.tugraz.at,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at,
        Andrew Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>, x86@...nel.org,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page
 tables (core patch)

On Fri, Jan 5, 2018 at 1:14 PM, Jiri Kosina <jikos@...nel.org> wrote:
> On Fri, 5 Jan 2018, Dave Hansen wrote:
>
>> >>> --- a/arch/x86/platform/efi/efi_64.c
>> >>> +++ b/arch/x86/platform/efi/efi_64.c
>> >>> @@ -95,6 +95,12 @@ pgd_t * __init efi_call_phys_prolog(void
>> >>>           save_pgd[pgd] = *pgd_offset_k(pgd * PGDIR_SIZE);
>> >>>           vaddress = (unsigned long)__va(pgd * PGDIR_SIZE);
>> >>>           set_pgd(pgd_offset_k(pgd * PGDIR_SIZE), *pgd_offset_k(vaddress));
>> >>> +         /*
>> >>> +          * pgprot API doesn't clear it for PGD
>> >>> +          *
>> >>> +          * Will be brought back automatically in _epilog()
>> >>> +          */
>> >>> +         pgd_offset_k(pgd * PGDIR_SIZE)->pgd &= ~_PAGE_NX;
>> >>>   }
>> >>>   __flush_tlb_all();
>> >>
>> >> Wait a sec...  Where does the _PAGE_USER come from?  Shouldn't we see
>> >> the &init_mm in there and *not* set _PAGE_USER?
>> >
>> > That's because pgd_populate() uses _PAGE_TABLE and not _KERNPG_TABLE for
>> > reasons that are behind me.

Oh, I completely missed that; and then the issue would have got hidden
by one of my later per-process-kaiser patches.

>> >
>> > I did put this on my TODO list, but for later.
>> >
>> > (and yes, I tried clearing _PAGE_USER from init_mm's PGD, and no obvious
>> > breakages appeared, but I wanted to give it more thought later).
>>
>> Feel free to add my Ack on this.

And mine - thanks a lot for dealing with this Jiri.

>
> Thanks. I'll extract the patch out of this thread and submit it
> separately, so that it doesn't get lost buried here.
>
>> I'd personally much rather muck with random relatively unused bits of
>> the efi code than touch the core PGD code.
>
> Exactly. Especially at this point.

Indeed.

>
>> We need to go look at it again in the 4.16 timeframe, probably.
>
> Agreed. On my TODO list already.
>
> Thanks,
>
> --
> Jiri Kosina
> SUSE Labs
>